During the initial fight between Piccolo and Imperfect Cell, Cell severely damages Piccolo's arm, seemingly leaving him incapable of fighting (at least, as far as Cell believes). After getting the information he needed, Piccolo regenerates his arm and the fight continues, much to Cell's surprise ("You tricked me! I should have known you were up to something!").

The problem with this scene is that Cell shouldn't be surprised by this. Remember, Cell is made from the DNA of several fighters, including Piccolo. So if Piccolo can regenerate, Cell should be able to as well (and indeed he does later on).

And even if you want to claim Cell doesn't know he can do this yet, Dr. Gero (Android 20) claims he had been collecting data on the shows various fighters for years, as far back as the 22nd World Martial Arts Tournament. This would have included the fight between Goku and Piccolo at the 23rd Tournament, where Piccolo used this exact same ability. Surely, if Gero knew Piccolo could do this, he would have made sure Cell knew about it when he programed him. So why was Cell so surprised by Piccolo's regeneration?