Besetting sin humorous noun phrase
A particular weakness or bad feature
She was so deeply in love with him that she is blind to his besetting sin.
Impatience is my besetting sin.
Used to describe a situation in which people criticize and blame someone else for a fault, mistake or weakness that they also have themselves
The origin of this phrase is not clear.
If you fall for someone's trick the first time then they are at fault.
Are you going to mess with me again? Fool me once, shame on you; fool me twice, shame on me.