I hate it when you're in a groove and the daily stand-up knocks you out of it. Now I can't remember what the hell I was even doing before it.

I agree. I have attended many such ad-hoc meetings and after the meeting was over, I forgot what I was doing

My trick when someone drops by to interrupt you, or some meeting starts soon and I have to leave: don't say a word, finish writing what you were writing, finish thinking what you were thinking and then the most important: think about what you're going to do after the interruption (e.g. what were you doing and thinking about before). Only then allow the interruption to catch your attention.

My coworkers understand that if I don't immediately reply (but I acknowledge their presence, raising just a finger works like magic) that I'm finishing whatever I'm currently doing.