Was thinking of barrels. We all know the awesome one that Adele came up with: It's like shooting fish in a barrel full of monkeys.
Well, these are probably just as true:
One rotten monkey spoils the barrel
One rotten apple spoils the barrel of laughs
The other day I said "My life is an open book you can judge by its cover."
And I have a nagging feeling that this is a repeat, but "He just pulled the sheep in wolf's clothing over your eyes."