Inspired by my DP just digging through his packet of crisps to find me all the folded-over ones. They're the whole point of crisps, for me – the ones that are folded in half. He now passes them to me without even thinking.
Similarly, if anything needs clingfilming in our house, I'll do it. Pulling the film off the roll and cutting it on the box blade sends DP up the wall – he ends up furious, sweating, and with the clingfilm all in knots. Hugely capable in every other way, but clingfilm is his Krypotonite, so we don't even talk about it any more.
What weird little habits have you developed in your relationship that are unique to you?