Fly

The idiom a fly in the ointment is meant to describe a minor flaw or problem that spoils an otherwise excellent situation.

~ Candace Osmond

- - - - -

That idiom doesn't make sense to me. Doesn't it equate and "excellent situation" with ointment? Although if you have a rash and the ointment helps, that probably is pretty excellent.

Also, I don't particularly care for the word "ointment."