I'm lazy, and wanted to see if I could add [or] to my favorite tags so I wouldn't have to type it in the search. I know it's not an actual tag, but I was curious.
Searching for just [or] gives results, and shows "Questions tagged or" under the question count.
Clicking the about » link takes you to https://meta.stackoverflow.com/tags/or/info which doesn't exist.
[tag1]
OR[tag2]
. This is probably due to the "or" search parameter being parsed as a tag, even though it's not.