In "Homer Goes to College," Homer gleefully sets his high school diploma aflame, while singing, "I am so smart, I am so smart, S-M-R-T, I mean S-M-A-R-T!" Behind him his living room is going up in flames.
The now legendary "S-M-R-T" line was actually not in the script and was a genuine error by voice actor Dan Castellaneta. However, the error was so much in Homer's character that they chose to include it in the finished product.
People now chant either "S-M-R-T" or "I am so smart, S-M-R-T", particularly when they're feeling stupid, celebratory, or both.