A "Simpsons" producer took a rare public swipe at "Family Guy" creator Seth MacFarlane.

At the long-running comedy's Comic-Con panel, a fan asked a question referencing MacFarlane's three successful shows that bracket "Simpsons" on Fox's Sunday night lineup ("Family Guy," "American Dad" and "The Cleveland Show").

"Seth MacFarlane has one show three times," said "Simpsons" writer/executive producer Matt Selman.

Fans gave one of those "ohhhh-you're-causing-trouble" noises.

"Simpons" creator Matt Groening didn't react, though earlier referenced MacFarlane himself when asked whether he would choose the "Simpsons" or "Futurama" if only he could have created one show.

"American Dad," he said.

Asked about Selman's line later that afternoon, MacFarlane dropped into a noble-sounding character voice and said, "While I disagree with what you're saying, I'll defend to the death your right to say it." (Didn't expect a Voltaire reference in this story, did you?).

More from the panel:

-- "Facebook" CEO Mark Zuckerberg will guest voice on the show next season, along with Daniel Radcliffe, Jon Hamm and Cheech and Chong.

-- Former "Simpsons" writer Conan O'Brien recently turned down an opportunity to appear on the show. "We invited him a few months ago but he had a lot going on," said exec prod Al Jene.

-- As usual, there's no plans for another film, or to wrap up the series.