A curate's egg
Something bad that is called good out of politeness or timidity.
The origin of the phrase is the George du Maurier cartoon "True Humility", printed in the British satirical magazine Punch, on 9th November 1895. The cartoon gives fuller insight into its meaning, which relies to some extent on an appreciation of irony.
Right Reverend Host. "Im afraid you've got a bad Egg, Mr. Jones!"
The Curate. "Oh no, my Lord, I assure you! Parts of it are excellent!"
Clearly an egg which was partly bad would be entirely unpalatable. The curate, being too timid to complain to his host, looked for something positive to say in reply. That initial meaning of the phrase; to describe something which partly good but which was ruined by its bad part, is now rather lost. That's not a situation that occurs very often. It's now more often used just to describe something that is partly good and partly bad; for example:
"How was your day?"
"A bit of a curate's egg. I got that report finished on time, but the car broke down as soon as I left the office."