Misuse of the MessageBoxes, especially when it in a loop. Thunderbird is a pain for this, when its processing message filter. Plus a lack of information to fix it. Which message did fail to send? Which Filter? etc
generate some form of report at the end with all the messages, or a some of come-back to list.
Another is Web-Pages that suddenly jump to content down the page, I'm reading that. Auto-activation of media should be banned, nothing worst then multiple ads shouting their wares at you. My mouse has buttons, should I want to watch it I'll click.