When Serafin is dancing to "Nina" one of the dancer's loses her flower in her hair. It is visible on the gazebo floor in one shot, then it's gone and reappears in her hair in a later part of the dance routine.
When Serafin is walking the tightrope to Manuela's balcony, the support wires can be seen.
When Manuela and Serafin first meet near the sea, the seam of the backdrop is visible.
When Serafin trips one of the fleeing guards with his whip, a white donkey crosses between them. A thin wire can be seen attached to the animal's halter to guide it.
During the the scene where Judy is pummelling Gene with objects her gold braid comes off you can see it dangling, in the next scene it's back in place.
The film is set in 1830. Manuela's trousseau is labeled "Maison Worth, Paris". The Maison Worth couturier house did not exist until circa 1860.
When Gene Kelly first shouts 'MACOCO' during the fight with Don Pedro, his mouth is wide open but he never moves it to make the third syllable of the word.
When Manuela's hat is blown off by ocean breeze, wire pulling it are clearly seen.
At the hanging, the Viceroy shows Manuela Serafin's chest filled with Mack the Black Macoco's ill gotten treasure. Manuela had been given her engagement ring by Don Pedro Vargas/Macoco. All she had to do was to show the Viceroy the ring on her finger and the matching bracelet that she finds in the chest to explain that the ring matches the bracelet and that Don Pedro Vargas, the mayor had given it to her, not Serafin.
Manuela is nowhere near the mirror to get hypnotise she is acting.