Don't provoke damaging plays on the second line
This is from the Bermuda triangle page. is weak - Black should now consider adding a stone at a, to play lightly here.
Is there any way Black can make this position worse?
is a bad idea. Now Black's territory on the side, his only gain here, is under threat. is a classic heavy move, since now cannot readily be sacrificed. It is also in a poor direction since Black cannot hope to close off the edge at both p and q. This can also be looked at as a variation of Squeezing out the Toothpaste.
This type of mistake seems to be very common.
This is from a nine-stone game, Black being a beginner. is a related kind of mistake. Even though Black defends everything up to , is a chance to attack.
Instead of , Black a is good.
Ironically, plays on the second line can be bad also. Nothing is ever clear cut :) Please look at Don't descend, connect