The joke’s on somebody
Definition 1 of 1
Pro Tip 1/3Self-Defeating Humor
This phrase often highlights an ironic situation where someone's attempt to be funny at another's expense backfires on themselves. 
He tried to scare his sister with a mask but tripped and fell; the joke's on him.
Pro Tip 2/3Unintended Outcome
Use when someone's plan or action has the opposite result of what was intended, especially in humorous or playful contexts. 
She hid her brother's phone as a joke, but he found her secret diary instead. The joke's on her!
Pro Tip 3/3Schadenfreude Caution
It warns of the possibility of finding oneself in an embarrassing situation after trying to embarrass someone else. 
Attempting to trip his friend, he ended up sprawling on the floor himself. The joke's on him!