When Joe is dancing in the Olvera street sequence he grabs two candles, blows out the flames, then uses the candles as drumsticks hitting some glass balls and other items. The sound given off is of a hard object striking the balls showing that the "candles" are wooden fakes since a wax candle would make a different sound.
While Joe Brady is sitting at the bar as the bartender keeps feeding him glasses of whiskey, he drinks the first and second glasses while talking to Clarence. The barkeep slides him a third glass, then a fourth. However, he never drinks the third glass. It simply disappears.
At the beginning of the fantasy sequence Gene Kelly falls into a tunnel. When he gets up there is considerable dirt on his white trousers. A few seconds later, when he leaves the tunnel, his trousers are clean again.
During the opening sequence the Navy band is supposed to be standing on the deck of an aircraft carrier. As the camera pulls back it is obvious that most of the deck is simply painted on and that the "deck" ends about twenty feet in front of the band.
During the "Spanish Number" the bandit (Gene Kelly) needs to swing across from the wall he is on to the roof. There are two red curtains hanging nearby to the right. He grabs the one on the left and climbs higher on the wall. As he does the upper end of the curtain moves more to the center and elevates by about ten feet, allowing him to swing across.