Fool Me Once, Shame On You proverb
The full version of this proverb is "Fool me once, shame on you; fool me twice, shame on me."
If someone pulls a trick on you the first time, they are the ones to blame. But if you fall for the same trick the second time then it will be your fault.
Don't even try to trick me this time, Robert. Fool me once, shame on you.
I can't believe I fall for his trick again. It's totally my bad. As they say "Fool me once, shame on you; fool me twice, shame on me."
This phrase was first recorded in a book called The Court and Character of King James by Anthony Weldon, 1651:
“The Italians having a Proverb, ‘He that deceives me once, its his fault; but if twice, its my fault.'”