PlanCacheClear::clear, two checks happen: one for whether the cache entry exists, and another for whether removing it was successful:

https://github.com/mongodb/mongo/blob/56a4d894cc679db1ccd2336c490a1c3cd838f0d6/src/mongo/db/commands/plan_cache_commands.cpp#L313-L325

This strange because the call to contains() and remove() each acquire the plan cache mutex separately, so it's possible for something to happen between the two calls and for the command to return an error without logging a message. While this won't crash the server or do anything really nasty, we should fix it. This would just involve removing the call to contains().

I do not believe we'd need to add any specific testing for this.