I don't think I've ever threaded one myself, though I do remember them in the classrooms. I recall how the end of the film would flap around when it was done. It was the teacher that touched those so we never did.
I also remember overhead projectors with transparency paper, I imagine they don't really use those anymore? I recall the really old models that were green, then they got the 3M models that were new and high tech looking. There was also another type of projector where the paper did not even have to be transparant. Even to this day I have no idea HTF that worked. Freaking voodoo magic! I guess it's something to do with bouncing light off the paper but still, how did it get such a clear image. I'm going to go with voodoo magic, just like RF. Those machines were huge and not used very often, also green. Green seems to be a classic colour for older equipment. Our DMS upstairs is green and brown. Brown is kind of the classic Northern Telecom colour.