D'OH!

Bart's Dog gets an F: After his idea for Lisa to do his homework backfires
Radio Bart: When Bart realises his name is on the radio down the well
The Front: After he loses to Rock-paper-scissors to Lisa
New Kid on the Block: When he says "I fell on my bottom" in front of Laura
Duffless: After Lisa finds the report she made about him with ease
Homer loves Flanders: When Bart realises being a loser in the Simpsons is genetic
Bart of Darkness: After his leg, already stuck in a bucket, gets stuck around a coil of hose
Lisa's Rival: When his prank call to Skinner backfires
Itchy and Scratchy Land (with the rest of the family) as the car pulls up to a traffic jam
Bart vs Australia: When he realises Lisa was right about the way water goes round the toilet in Australia
Bart's Girlfriend: When Marge tells him Jessica will be at church
Raging Abe Simpson and his Grumbling Grandson in: The Curse of the Flying Hellfish (with Grampa) When Bart and Grampa realise the treasure is at the bottom of the ocean
The Canine Mutiny: When Mr. Mitchell discovers Bart is trapped in the closet
A Tale of Two Springfields: When Homer gives the guards the chloroform, instead of using it on them like Bart suggested
The Great Money Caper: When he tries to escape but still gets arrested