Calling with a small pair aiming only to hit a set.
You flop a set about 1 time in 8 (the math alone says 7.5×). You won't always win their whole stack when you hit, so the opponent needs a stack of 15–20 times your call. For a 3bb call, 45–60bb or more must be left. If not, even hitting the set won't pay back the cost.