Well I have seen it happen a hand full of times. The zilla I just bought had this problem the impeller started chewing up the cover and the bolt was almost all the way backed out. I have seen this on other ATVs and dirtbike as well. Reverse threads help prevent this but it isn't full proof. That is why the manual calls for lock tight.

Had a few like this, last one, the impeller was spinning on the shaft.
LOCTITE!!!