the joke is on someone

US /ðə dʒoʊk ɪz ɑn ˈsʌmˌwʌn/
UK /ðə dʒoʊk ɪz ɑn ˈsʌmˌwʌn/
"the joke is on someone" picture
1.

used to say that someone who has tried to make another person look silly has in fact been made to look silly themselves

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He thought he was being clever by tricking me, but the joke is on him now that I've exposed his lies.
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They tried to embarrass her with a prank, but when it backfired, the joke was on them.
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