For me, it has to be when people move things, and then don't move them back to their original place. I find it incredibly annoying, and forces me to go out of my way to properly place the things back, I also get mad.
[spoiler][b][i][u]KUPO!!![/u][/i][/b][/spoiler]
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I don't know if this would be classified as annoying (depends who you are I suppose) but I never use abbreviations like "tbh," or "lmao." I'll use lol but that's it. Anything else I will [u]always[/u] write out completely.