In the Season 4 episode of Arrested Development, Tobias is caught by a To Catch A Predator style show when he thinks he's going to meet Maeby at home. It's not clear from the episode how he became the target for the show, unless I've missed something important. Is this just a coincidence, or explained in a later episode? Is there even an explanation?
It was simply a matter of Tobias being in the wrong place at the wrong time. He walked into the sting operation and said:
"Is there a little girl in here? Daddy needs to get his rocks off."
He's simply referring to his daughter when he says "little girl", and is indicating he needs help getting his rock suit off, as he was impersonating The Thing. Unfortunately, it came off as he was actually entering the house looking for a little girl to "get his rocks off", which is an idiom for "ejaculate".
As such the crew thought he was entering the house to have sex with a little girl, as they were set up to run a sting operation to catch pedophiles. Tobias had no knowledge of this prior to entering the house.
Tobias has a history of saying double entendres, where he means one thing, but what he said has another, often sexual connotation that someone might think he means instead, but Tobias never realizes this.