After its second season, Ashton announced that he was ending the show, and the media went crazy. When the show debuted its third season, he announced that it had all been one giant punk to the public.
During the first season, at the end of every prank, Ashton Kutcher would run out and say to whoever got "punk'd", "You've just been punk'd! you're on my new MTV show!" By the second season, he didn't need to say anything. His presence was a sign to whoever got punk'd.
Ashton Kutcher jokes that he's not allowed to be "punked", but in fact it has happened twice - the first time was a test run by producers before the show aired (he was constantly asked to walk through an X-ray machine at MTV offices). The second time occurred when Britney Spears got Kutcher's staff to turn against him. But plan backfired. Ashton realized what was going on, and got Britney to turn on his Punk'd crew.
According to Kutcher himself, he failed to "punk" Neve Campbell twice. Kutcher finally tricked Campbell on his third try and eventually made it to the Hall of Shame Standard Edition.