heat from the projector lamp makes the film expand in its mount, distorting it, pushing it out one direction or another. then your image is out of focus on the screen. you may be able to re-focus, but maybe only part of the image will be in focus.
I haven't had a big problem with it. I figure if the slide's been in there for 15-30 seconds and it "pops", time for the next slide. but it depends on the application, thus my question as to what you are trying to accomplish.