When Robespierre gives Mewsette the idea to go to Paris her nose disappears for a few frames.
Jaune-Tom and Robespierre set off from a station in Provence signed "Moustier St Marie". It is a real place but spelled "Moustiers Ste. Marie"; the Virgin Mary is after all always in the feminine.
Invisible ink will always reappear, on any document written with it, when carefully reheated. So, the check to Mme. Reuben-Chatte might still have been cashable.
The film supposedly takes place in the 1890s, but the montage of painters doing portraits of Mewsette includes Vincent Van Gogh and Pablo Picasso. Van Gogh died in 1890, while Picasso would not exhibit in Paris until 1900 (and would not paint in the style depicted until the 1920s).