The music mogul turned 54 on 7 October and to mark the occasion, US X Factor band Fifth Harmony draped a giant birthday flag across the front gates of his LA home.
A nice gesture, you might think…
But the cheeky fivesome of Fifth Harmony - Ally Brooke Hernandez, Normani Kordei Hamilton, Dinah Jane Hansen, Camila Cabello and Lauren Jauregui – decided to wish Simon a happy 87TH BIRTHDAY rather than 54th. Naughty!
They also decorated some of the trees on his lawn with toilet paper.
After seeing the "gift", Simon tweeted: "Verrrrrrrrrry funnnnnny @FifthHarmony."
Fifth Harmony added: "LOVE YA SIMON!!! Happy birthday."
Plenty of people seemed to have a giggle about the prank, including One Direction's Niall Horan, who tweeted: "Good job girls! that's so funny! see ya soon ladies! Xx
The band replied: "Hahaha thanks! Miss you… WE SHOULD TP HIS HOUSE TOGETHER SOMETIME."
Simon, who is expecting his first child with girlfriend Lauren Silverman, adopted two puppies as a birthday present.
He later tweeted: "And for anyone thinking of buying a dog. Do it. They bring so much happiness into your life."