You turn it on while holding it backwards.
You make that sharp crackling noise with your mouth each time you clash it with your opponent’s lightsaber—having forgotten that the noise happens naturally.
You’ve given in to the Dark Side of the Force, so the beam is normally red. But you forget to replace the weak batteries in the thing, rendering it pink, and turning you into the laughingstock of the Empire.
You try to use it to cut your birthday cake, expecting the lightsaber to slice through the pastry as easily as it did Luke’s hand. Instead, the cake vaporizes the instant the lightsaber touches it, à la Obi-Wan.
You mistake it for a Popsicle.