Other mistake: During the dance class, the girls slip on the water and fall over despite none of them actually standing on any of it. (00:12:21)
Other mistake: After Nani kicks the car's bumper and calls the driver a stupidhead, it's shown that the Pelekai house has exterior railing at the front staircase and balcony. While Nani's head is inside the dog door as she shouts at Lilo, when Mr. Bubbles walks up the stairs and nudges Nani's foot, at this particular angle part of the handrail or baluster should be there, but is deliberately missing in this shot. (00:15:20 - 00:16:00)
Other mistake: While Nani is in Lilo's bedroom, they both see the "falling star" outside, and when Lilo tries to push Nani out of her room so she can make a wish, the left wood banister is deliberately missing from where it should be. (00:21:05 - 00:23:15)
Other mistake: Stitch watches 1958's "The Spider" on TV in a store's window. Some shots face the TV screen, and other shots face Stitch so we're seeing the TV screen's reflection. When Nani tells Lilo they'll meet at 1:00, in this shot facing Stitch the reflection of "The Spider" movie is properly backwards (note "mattress" sign), but two shots later the reflection of the woman screaming in that movie is in fact not reversed as it should be ("The Spider" scene timecode is 00:55:50). (00:28:50)
Other mistake: When Nani sees Stitch standing in front of the open fridge, Stitch's shadow behind Nani is of his alien form, with four arms and two antennae. When Stitch is actually shown, he is in his "dog" form, with two arms and no antennae. (00:39:12)
Other mistake: The fire truck passing by Nani on the way to her destroyed house has the same license plate as the gas truck that Stitch hijacks.
Answer: There are two times when this happens the first time he is holding his guns. The second time he is holding a frog.