#3: buttons that reveal themselves on hover.GMail mistake
The most common problem with that is that you don’t see the button in advance, meaning you can’t move your mouse directly to it. Instead, you have first to move your mouse to reveal controls, and then decide/correct after they appear. Two moves instead of one, big deal when it comes to the interfaces you use every day. Notion, as one example, has the same problem grumpy.website/post/0PhAYfy0b
But GMail made things even worse (being grumpy, I wouldn’t expect anything less from them). They put on-hover controls in a dense and _active_ area of an email card. To open any email you _need_ to click anywhere in that small rectangle. This is the _primary_ interaction, this is the one and only way to open emails and switch between them. But—surprise-surprise!—when you try to do so, accidental hover control might appear right under your mouse out of nowhere. Those hover buttons even overlap the text!
The worst part? Those controls are not even that important in the first place. You can do the same action after you’ve opened an email, or select it and do it from the top menu. In that case, GMail team tried to save you a click where you didn’t even asked for.