Revealing mistake: When Anakin and Padme's shuttle arrives at Naboo it flies over a lake and lands on the other side next to some trees. Even though the trees are visibly reflected in the lake, at no time is the shuttle ever reflected on the lake when it passes over it and lands by the trees. Bizarrely though, its shadow is always visible.
Revealing mistake: During the scene at the dinner table with Anakin and Padme, Anakin makes a piece of fruit fly around. He then has it land on a plate and cuts it with a fork and knife. It happens fast but you don't really even need slow motion or pause to see that the fruit doesn't react to Anakin sticking his fork or knife in it at all. The knife and fork just vanish into the fruit.
Revealing mistake: When Obi-Wan is cut by Dooku's lightsaber and falls, you see one of the wounds (the one on his arm) glowing and then suddenly seem to shrink and disappear, though not totally. This is because it is computer generated and didn't match up exactly with the actor portraying Obi-Wan. Hard to catch, but it is there.
Revealing mistake: When Anakin and Dooku are crossing the hangar shortly after Anakin's green lightsaber was destroyed. Anakin slices a thick cable on the ground causing the lights to flicker. Even at normal speed, it is blatantly obvious he never actually hits the cable, and furthermore the pyrotechnic effect is slightly out of synch with Anakin's motion.