Fafo, a stripping, middle-aged boy scout that left everybody half Squick and half laughing their asses off.
Sharon actually had one of these in a YouTube special, when a magician hurled a piece of "used" dental floss at known germophobe Howie (see here to get the full effect), sending him scrambling away from the desk. Sharon was laughing so much she could barely talk for the rest of the segment.
A bit of a stealthy one, but in the final elimination for Season 5's semifinals, Howie directed the audience to the three acts already making it in and noted they looked like the normal American family. Cue Prince Poppycock putting his arm around Michael Grimm.
Tons in season 6, mostly due to Howie and Piers constantly annoying each other.
Howie's pranks during the auditions - putting the vending machine in front of Piers's dressing room door, or filling his dressing room with Those Funny Little People.
Those Funny Little People and Piers. He hates them, and they love him for it, further annoying him.
During the finals the judges each got to bring back four elliminated acts to perform again. Howie chose them as one of his, and found more amusement in Piers facepalming while the act was going on than the act itself.
During the Top 48, week two, Howie pulling a small water gun out of seemingly nowhere to spray Piers with.
Then week four, he brings an airhorn.
Then there's the whoopee cushion Howie puts on Piers' seat (which fails), Howie cutting Piers off in the middle of a critique using his buzzer...Really, this season might as well be renamed "Howie Trolls Piers".
A triple Crowning Moment of Funny, Awesome and Heartwarming, Landau Eugene Murphy, Jr. and Anna Graceman are called on stage to see who goes on to the Top Ten. Anna gets the win, she and Landau hug and in his exit interview with Nick says he's just grateful to have come as far as he has...then Nick reveals that Landau got through too. Cue Landau picking up Anna in his arms and spinning around hugging her.
Doubling as a Crowning Moment Of Awesome for anti-fans of Piers, J. Chris Newberg of Season 6 roasted Piers, who buzzed him in all previous performances and declared he wasn't funny. The roast itself was good and was as hard-hitting as Piers himself tends to be, and Piers agreed that it was very funny, but the topper was Sharon's reaction at the end.
"You've got balls of steel!"
Howie pulling the fake snake out of his pants, scaring the life out of Mel B and Heidi.
In Season 9, the show's teasers before the commercial built up a bad mime that would confront the judges at the desk prompting Howie to call security. Come the act, he wasn't very good, got four Xs, and was booed. He then began to yell at the audience and insult the judges before angrily coming down to the desk to confront them. At which point it turned out to be host Nick Cannon playing a joke. The judges couldn't stop laughing.
Also from Season 9, Aaron J. Field comes on stage wearing a giant baby costume with a hat and a large pacifier hanging around his neck, after the show kept showing teasing clips of him in costume beforehand. Turns out he's a singer, and a legitimate one; the costume was just to get attention and make himself memorable to the judges.
Two guys start performing "I Gotta Feeling" and aren't good resulting in the audience booing and despite the judges hitting their buzzers, they got no X's. Maintenance had to stop the act, turns out the reason the buzzers weren't working was because Mel B had unplugged them to charge her phone.