As the Sergeant does a backbend while Mabel sings to him, the position of his hand on his chest keeps changing between shots (as opposed to the actual movement prescribed by the choreography).
Leap years don't occur in century years unless the year divides by 400, hence 1900 wasn't a leap year and therefore Frederic wouldn't have reached his 21st birthday for 67 years, not 63.
The lip-synching is often off (or ceases altogether), especially in choral numbers like "When the Foeman Bares His Steel".
About 40 minutes into the film, while Mabel sings "Poor Wandering One," a bird flies behind Frederic and hits the "sky" backdrop, causing the bird to fall to the ground. This is only visible if watching the film in its original aspect ratio of 2.35 to 1. For television airings which have modified the dimensions of the film, the bird is still shown but you do not see it hit the backdrop.