In the flashback section where Humphrey Bogart and Ingrid Bergman and that guy who plays Sam are in the club in France, it shows the shadow of the words on the window. The shadow is inside, so the words should be backwards, but they are forwards! No biggie, but still.
THIS IS NOT AN ERROR!
If an outside light source creates an interior shadow because of lettering on a window, the shadow will look exactly like the lettering does as viewed from the outside of the window. In this scene, the shadow is correct. The producers would of had to defy the laws of physics to create this mistake.
Sorry Rapunzel, it aint Armstrong who plays Sam, it's a guy called Dooley Wilson. An interesting note of casual racism; When Bergman asked the waiter who was playing the piano, she asks who is 'the boy', even though Sam is a lot older than her.