I propose to add tag "error" (synonym "mistake") for questions like Category theory - where is my error?
This is a meta tag, it is about the type of question and not about the topic of the question. There are no experts in the tag of error, and there is no reason to use it as a favorite tag or to search for questions with that specific tag.
Tags work best if they are about the topic of the question, if they are useful to select questions fitting to your expertise, or if they can be used to find questions. If a tag is not useful in this way, it is often a bad idea to use it at all.