Always check the bets that you have placed (as long as no time constraints like trading). It sounds so obvious but on multiple occasions I have thought a bet has been placed when it hasn’t.
I might have backed the wrong player or team, a selection in the wrong market, bet just not gone through. Lots of different things. If I am using bookmaker accounts then I will take a photo of the bet.
I am seeing it increasingly on Betfair where you are not sure if a bet has been placed. Often you have to go out of the market and come back into it to make sure the bet is right.
The worst example I can remember was when I was having a 5 figure bet on a football match. It was placed twice by accident but I was only seeing it once. Obviously lost and is utterly maddening.