Are there certain tags that are classified as invalid tags (and if so, what are they)? Some of the images I've been tagging have gotten the invalid_tag tag. So, I just wanted to know which tags to avoid using to avoid the invalid_tag tag.

There is a list available, but in practicality you're not going to memorize every one.
invalid_tag

basically the tags that have been invalidated are troll tags, trope tags, and generally tags that cant really be visually placed in images as they are to ambiguous