To call 3-bets with low pocket pairs. This is a mistake that is committed by the majority of players at lower levels. Suppose that with 100BB stacks, we raise 44 to 4xBB and get 3-betted to 16BBs. Here, most players would call with the reasoning (in the best case, most would just call) that if we flop a set, we take all of Fi’s money. They might even be advanced enough to show a bit of mathematics behind the result; we need to call 12BBs to win partly what is in the pot (20BBs) and partly the 84BBs that Fi has left in his stack (implied odds). Our combined odds are therefore 104:12, or ~8.6:1. We know that the odds against the flop being a set are ~8.5:1.
We can therefore call. WRONG!!! For this calculation to be correct, we need to actually win all Fi’s money every time we flop a set, which we will definitely not be able to do. A fairly standard 3-bet’s HD is TT+ and AK. What happens, for example, if the flop is Q4x? We can forget about getting more than a c-bet from AK, TT, JJ. What if the flop is A4x? No money from TT-KK. We even have to factor in the possibility that we will both flop a set, even if it is remote. Without going into the exact mathematics, we can easily see that in a lot of cases we will not win those necessary 84BBs and if we call we will be playing an EV game that will lose money in the long run.
So, when can you call with a PP to flop a set? There is something called the 5/10 rule which says that one can always call 5% of one’s stack with a PP and be +EV. If you want to call 10%, you need mitigating circumstances (for example, your opponent is a big fish who only raises with AA and never folds them). Everything between 5-10% is a question of judgement and depends on the actual Fi’s game, his and your position, etc. My rule of thumb is that 7.5% and better is always a call.
Now, I’m sure a lot of people think it would be better not to raise with 44 from the beginning, to just call 1BB, let Fi raise to 5BBs and then call the remaining 4BBs, thereby investing a total of 5% of your stack. However, this brings us back to the mistake of calling first. The advantages of raising are simply so much more compelling that it is worth losing those 4BBs at times when we get 3-betted. The vast majority won’t start 3-betting more because one is aggressive, but rather begin to call more, which is absolutely fine with us. Should you meet someone who 3-bets more than he should be able to back up, the choice is simple: change table or sit fast and 4-bet sometimes so that he won’t think he just needs to pick up easy money. Changing tables is probably best, as we want to play against fish and not get into power struggles with someone who is as good/better than us; that’s not the way to win money.