My "favorite" aspect of the Slack web app, which might have been fixed, as I haven't seen it recently, is if you typed up -> r (I believe it's r) to open the emoji picker, typed a few characters (e.g. "eyes") and pressed enter. Well, if the emojis hadn't loaded by then (and apparently they're loaded fresh every time), they would pop in and often, the a different emoji would be picked than the one that was visibly highlighted when I pressed enter.