A judge declared a mistrial in a Philadelphia courtroom Wednesday when an alleged assault victim's prosthetic eye popped out while he was testifying and startled jurors.

John Huttick, 48, was sitting on the witness stand crying about the loss of his left eye that happened when he allegedly tried to break up a bar fight in August 2011, when the fake eye popped out and he caught it in his hand, reported the Philadelphia Inquirer.

The jurors gasped.

Matthew Brunelli, 23, is alleged to have punched Huttick in the eye with a car key between his fingers.

Defence lawyer Eileen Hurley alleges Brunelli didn't use a weapon and was acting in self-defence.

Hurley demanded a mistrial in the aggravated-assault case as she said the eye-popping accident could have caused the jury to have "undue sympathy" for the alleged victim and "undue prejudice" against the accused, the Inquirer reported.

A new trial was set for March 4.