Mine was 100% overplaying one pair in spots where the board is just screaming trouble. Took me way too long to stop marrying hands like AQ on A-high boards when the action got silly on the turn. Sounds obvious when you say it out loud, but at the table, especially after a long session, you can still talk yourself into some bloody average calls.
These days the biggest edge I reckon most regulars get at small to mid stakes isn't from fancy stuff, it's from not punting in dumb spots. Folding weak top pair earlier, not defending rubbish just because you're in the blinds, easing off the auto c-bet when the flop smacks the caller way harder than your range... that sort of gear. Heaps of players still burn chips by treating every decent-looking hand like it has to go to war.
Curious what took everyone else the longest to clean up. Preflop discipline? Calling 3-bets too wide? Fancy play syndrome? Bluffing the wrong player pool? Keen to hear the leaks that kept costing you for ages before it finally clicked, live or online, cash or tourneys.