Small blind
The smaller forced bet, left of the button. The worst seat postflop — first to act, half a bet already in.
Say it's a 1/2 game. You post the small blind, a dollar, before you see a card. It folds to the cutoff, who raises to 7. You look down at pocket eights. Calling is 6 more, and you're not just calling a raise. You're signing up to play the rest of the hand out of position against someone who acts after you on every street. Flop comes king-nine-four. You check first, no idea if the cutoff bets, no way to set the price. That's the seat.
The dollar already in the middle is bait, not a discount. New players see it sitting there and feel like they've paid for something, so they call raises they'd otherwise fold. That dollar buys nothing. It's gone whether you fold or play on. The only question is whether the hand and the price justify playing from the worst seat at the table for every street left. Good players fold a lot of small blinds, even to small raises, because acting first with no read on the raiser costs you again on the flop, turn, and river.
The mistake is treating the small blind like a cheap ticket to a flop. You're paying to play out of position for the whole hand, and the money you posted has already left your stack. Fold the hands you'd fold from any other seat facing that raise, and don't let a sunk dollar argue you into a bad call.