OTPs that are longer than 6 digits.
Bear with me for a moment.
6 digits is manageable to remember. You glance down at your phone for a second to read it, and you kinda just process it without even really reading the numbers. You naturally split it into two sets of 3 digits in your head, and it's not that hard to remember it that way for a moment. Then you type it into your computer, and you get it right. It just works.
But then along comes a company who thinks they're more secure than everyone else, and they text you an 8-digit code. Good luck glancing at that and then typing it in elsewhere. You're definitely coming back for a second look, either after typing the first part of it into your computer, or after you get the whole thing wrong and have to start again from scratch.
/rant