Star Wars Episode 3: Revenge of the Sith - Qui-Gon, May I Please Borrow Your Lightsaber?
When Agen Kolar, Kit Fisto, Mace Windu, and Saesee Tiin go to arrest Palpatine, Palpatine's last words before they duel are "It's treason, then." Right when he says that, he grabs his lightsaber.
Take a look at his lightsaber. Does it seem weird? It should, because its not Palpatine's real lightsaber. Palpatine's lightsaber has a cool silverish gold design on it. Adn this lightsaber is black and has a black handgrip with little grooves in it. This is Qui-Gon's lightsaber, as you can see if you watch Episode I.
Want proof that this is a true slip up? Ok. Towards the end of Palpatine's duel with Mace Windu, Palpatine very briefly raises his lightsaber, and you can see clearly it is Palpatine's own lightsaber. You can see this because of the silverish gold design on it.
By the way, if anyone wants the frame times for both of these moments, the one where Palpatine grabs the lightsaber is 01:08:34.
The one where he raises his own lightsaber during the duel is 01:09:58.