I've seen LevelledCompactionStrategy occasionally logging:
warnings from a ValidationExecutor thread.
<sstable_name> from level 0 is not on corresponding level in the leveled manifest. This is not a problem per se, but may indicate an orphaned sstable due to a failed compaction not cleaned up properly."
What's happening here is that a compaction running concurrently with the validation is promoting (or demoting) sstables as part of an incremental repair, and an sstable has changed hands by the time the validation compaction gets around to getting scanners for it. The sstable isolation/synchronization done by validation compactions is a lot looser than normal compactions, so seeing this happen isn't very surprising. Given that it's harmless, and not unexpected, I think it would be best to not log these during validation compactions.