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Let us look at the position in Fig.1. As we can see, the game started with two parrallel hoshis. After Black played a kakari against the upper right corner, White answered with a severe one-space pincer - a practice which becomes more and more popular these days. The two usual responces Black has to such pincer is to either jump to 'a' or to enter the corner - and this is exactly what he decided to do in this example.
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The common joseki which can be played in response to Black jumping into the corner after being pincered is shown in Fig.2.
          
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Sometimes White may continue by exchanging 'a' for 'b' before playing elsewhere. Sometimes, in simmilar positions, White might reinforce at 'c', but this usually happens with two-space pincers and three-space pincers. The shape after one-space pincer is strong enough so that reinforcing at 'c' might be just a loss of sente.
The result of this sequence is that Black gets some secure points in the corner, while White builds some outside influence directed mainly down along the right side, bot also towards the center. White also has the sente - which is usually used to stake out a position along the right - in this particular case, White can play a kakari of himself against the Black hoshi in the lower-right corner... of course, this kakari should be directed appropriately.
The separated Black stone still has some aji left in it, and many a merry fight has been started by trying to pull this stone out. All in all - this position is a joseki, which means that both sides should be equally satisfied, at least in the local context.
Please notice that White's influence is directed mainly along the right side of the board - this is important and we have to assume that it fits the overall board situation and corresponds to White's gameplan. Also notice the White move at '4' - it is a straight extension, and not a hane. This too is important - and if White tries to hane here, the position stops being a joseki and becomes a loss for White.
So, here we are - with The Blooper! The hane of '1' shown in Fig.3 is a bad move. It is surprising to say this because it seems so natural, and so common - you might have the feeling that you have seen it many times in many games. And this might even be true.
Nevertheless - it is a bad move.
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One of the inidces that the hane of '1' is bad is the simple fact that it is not joseki! We have seen the joseki sequence in Fig.2 and White definitely does not hane there! Such non-joseki moves should make you suspicious and laret - chances are these moves are somehow inferior and there might be a way to take advantage of this.
But, of course - such "unknown" moves might just mean that your opponent is more familiar with the joseki book that you are. So the right thing to do is to stay alert but not too eager to press any advantage - which might sometimes prove immaginary, hehe...
Nobody said this was easy...
Well, returning to the topic at hand, lets see why this particular joseki "deviation" is bad.
What is it so bad that Black can do to White after White plays the hane at 'x' in Fig.1? The key to the answer is the Black tesuji of '1' and '3' in Fig.4.
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Lets look at the most severe sequence - White falls into the trap with both eyes closed!! Look at Fig.3 - the key is the neat tesuji of Black '1' and '3'.
Lets look at some possible sequences - and lets see why they are bad for White.
| Position 1 | Position 2 |
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Look at Position 1 of Fig.5 - what a diseaster for White! The whole corner is lost, and what is left are a few dangling stones here and there, disconnected and in disarray, struggling to survive. This is uauslly the right moment for White to resign the game and go home to think what went wrong.
This is the worst case, the case when White falls fully into the Black trap. But what are other alternatives? What else can White do? Lets look at Position 2 of Fig.5, which shows the second-worst case for White.
Here White saw the danger a few moves earier and choose the "smaller evil" by sacrificing his stone on the top. The result in this case is not as drastic as in Position 1 of Fig.5, but nonetheless, White has suffered a big loss. Just compare this position with Fig.2 which is a joseki - and you will see that Black's position is much better - he has more points and influence (along the top and some towards the center) while not having any clear weaknesses - just the opposite, his shape looks stronger than in Fig.2. White's position is, conversely, much worse - he has less points (especially since the sente hane at 'a' is Black's prerogative), and the separated Black stone still retains some aji... although, admittedly not quite as much as the single Black stone in Fig.2.
All in all, compared to the joseki in Fig.2 Black's shape is better while White's shape is worse. This is a loss for White.
Another way for White to play this position is given in Fig.6.
| Position 1 | Position 2 |
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In this case White seems to have realized the danger early on, and so he prevented any of his stones from being captured. Black still got a position in the corner, but maybe a little smaller than in the joseki shown in Fig.2. So - should White be happy with this?
The answer is - No, he should not. And there are quite a few reasons for that.
The first reason is that after the sequence in Position 1 of Fig.6 White's influence is directed along the top side, and not, as in the proper joseki, along the right. This might or might not be bad, but as you can recall, we have assumed that to build influence along the right side fits the gameplan White has, so forcing him to face another way migh be a nice thing, at least psychologically. One can argue that if White wanted to have influence along the top, he should have blocked the Black sna-san invasion from the other side... thus resulting in the position given in Position 2 of Fig.6.
This is also a common joseki, one which gives White influence along the top of the board. Comparing the sequences from Fig.6, we can see that the White wall in Position 2 is positioned one line to the left as compared to the White wall in Pisition 1, and this means that if White decides to use this wall to build a moyo on the top, the moyo has a potential to be one line larger. This is small difference, admitted, but combine enough such small differences together and you'll win every game, hehe... Black wall in Position 2 is one stone "higher" than in Position 1, which is of some importance too.
Another reason why Position 2 of Fig.6 is not so good for White is that Black gets the sente!! This is very important! Remember - from the discussion of Fig.2, we assumed that White's gameplan was to build influence along the right side in sente, and due to his mistaken hane, all he got was influence along the top in gote! Now - this gotta be good for Black, no?
Also, please notice that White has some big defects in his shape in Position 1 of Fig.6 since Black can aim at the severe cut at 'a' at some point during the game, depending on the ladder. And even if the ladder does not favor Black, he always has the sequence of Black 'b', White 'a', and Black cut at 'c'. This is something White has to take into account in all his future plans. White has to be on his toes here, and this is good for Black.
So, how come White makes this hane in so many games? One of the reasons might be a misunderstanding of a common Go proverb - "Hane at the head of two stones is good". It usually is good, true... provided there are no tactical blows which can make it bad, like in this case.
But there are also other reasons why White might play such hane.
First of all, lets look at Fig.7, where Black simply invaded the corner.
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This is a standard sequence, happens all the time. White blocks at '1' and then hanes at '3'... This is the same hane as the one we are discussing here, and yet there is nothing wrong with it in Fig.7. Why is that? The answer is simple - Black cannot play the sequences starting with Fig.4 because he does not have the important support stone on the outside! Without this stone, the tesuji of '1' and '3' from Fig.4 does not work - it is not a tesuji at all, but a mistake!
Anyways - this kind of sequence is what White has in mind when he plays the hane at '3' in Fig.4. And if Black is not aware of the intricacies of the position, if he does not know how to use his outside stone properly, he will answer the way White expects him to... the resulting position is given in Fig.8.
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The question if White position is better in Fig.8 than in Fig.2 might well be a matter of oppinion for many players. One might argue that Black is pressed towards the edge more in Fig.8, and so his teritorry is smaller. On the other hand, White has the bad aji of the cut at to worry about in the futurs, and due to his stones being further up, the separated Black stone retains a littl more potential than in Fig.2.
But trust me when I say that every strong player will prefere Fig.2 over Fig.8 for White. It's that simple.
To summarize - As already mentined, the sequence from Fig.7 is a standard sequence. It might be one of the very first sequences a beginner learns. Together with its hane at '3'. And this is the hane people remember and unthinkingly apply to other positions. Probably with the idea that this hane is much more "severe" than a straight extension. And it is more severe indeed - but with opposite polarity.
So, in a sense, such bloopers can be traced to players' misunderstanding of common joseki moves and their failure to keep their josekis apart... as in "The Case of Misplaced Slide", this Blooper has its roots in people mixing two simmilar, yet different, sequences.
Keep your stuff straight, and you'll be fine. Hehe...