Bruh
2000 Reasons why you lost on time


You were one move away from checkmating your opponent so you did a victory dance and came back to your seat in time to see the one second left of your time slipping away to zero
You were playing in a team match and your opponent offered a draw. You went to check the remaining games to see if a draw would win the match for your team and either <A> spent a lot of time realizing you had to go for the win and had only seconds left or <B> flagged while you were analyzing your teammates positions.
Your opponent flagged and you called to let the arbiter know. The arbiter came up behind the clock picked it up, turned it around to see it and saw your opponent's flag showing on the side of the clock that was now closer to you and declared that you had flagged while ignoring the dozens of witnesses that said he'd made a mistake (Reshevsky-Denker in round 6 of the 1942 US Open Championship).
You went to get an arbiter so that you could claim three-fold repetition but you did not stop the clock. While walking back to the board with the arbiter you both saw the flag fall. The arbiter said that since you made the claim while the flag was still up the claim would be resolved first. Since your claim involved a position where en passant was possible the first time (it was not three-fold repetition with every piece having the same options) that meant your claim was denied and you lost on time.
The president of Fide called to wish me happy birthday, and I ran out of time while talking to him.