This one stumped me for literally like 3 years, before I finally found the answer online. Got it from the House of Leaves forums, but it’s older than that:

A man is walking in the city around lunchtime. He sees a restaurant with a board outside advertising the special of the day, shark fin soup.

He walks in and is seated. When his waiter comes by, he tells him, “I’ll have the special.” The waiter brings the soup. The man takes a bite, then freezes, a horrified look on his face. He bolts from the restaurant, pulls out a gun and shoots himself in the middle of the street. Why?

Hint: He still wore his wedding ring.