I think the biggest mistake a player can make is not adjusting to his/her opponents. Rather than applying a "one size fits all" strategy, you should be modifying your play based on who's at your table.
Also, registering for a tournament and forgetting to show up is pretty dumb too.
several errors are candidates to be the worst error ,,, playing unconcentrated, not analyzing what is happening at the table at the moment, playing with fear, playing with sleep, I think it is a set of errors, that is why I think that several errors are candidates to be the worst mistake
I think that one of the most expensive mistakes that players make very often is when a player bluffs without knowing how to do it !!! Sometimes it looks even funny)))
going all in with 100% of your bankroll without looking at your cards. also mucking your cards instead of turning them over, you'll be surprise. also if board is AAAA2 and opponent all in and you have 2,3 and you call. that would be pretty bad. over betting and giving away chips instead of small bets to get a feel at where you and your opponents are at.
Poker has it's probabilities(expected value), example: AA preflop, EV is favorable(80%).
The worst thing is when you catch a few cards, and think it's magic, EV is in your favor today. Also, another bad mistake is the theory of "due"; I've lost a lot, I'm due to get some good outcomes, to balance it out.