Usually, it is lack of concentration, not lack of knowledge, but there is an auction type that seems to cause trouble.
In my classes (usually on play of the hand), I innocently put in deals such as this:
North deals and the auction should be routine.
North opens 1.
South responds 1 (though some students get mixed up and think this is a 2/1 GF auction and respond 2--which of course, is a mistake. 2 is a jump-shift. Not a 2/1 bid. 1 is normal, showing 6+ HCP).
North has a routine rebid. He should raise spades. He has a minimum opening bid, so should raise, of course, to only 2. Yet, the rebid is often something else. Some raise to 3 thinking it shows a limit raise. No! A raise to 3 shows a strong hand--in the 16-17 invitational to game range. Others do even worse, raising all the way to 4! This shows 20 or so points. Remember that responder might have only 6 points.
So, I decided to write a little quiz. In the questions below, your job is to answer if the last bid shown is more likely to be 13 points (approximately) or 20 points (approximately).
A) 20. The response of 2 could have been made with not much (like 6 or 7), so the jump to game is very strong.
B) 20. East's 1 didn't promise more than 6 points, so West must have about 20 to insist on game.
C) 13. East's partner opened, so East is just insisting on game with his approximate 13. With 20, he would have done something stronger to explore slam.
D) 13. Same explanation as C.
E) 20. Same explanation as B (the negative double on the 1-level could have been made with only 6 points).
F) 13. When already in a game force, a jump to game is a minimum in context. Contrast this to A (where you weren't forced to game)
G) 13. Same explanation as F. Go slower with extras to explore slam (when already forced to game).
H) 13. An easy one to end.
Updated: June 2021