My worst torment training was in the early 90s where we had a lovely laptop that you could fold the display back and put it on an overhead projector to use the light and lens of the projector to project onto a screen, before the days of decent portable projectors.
Except...
The LCD thingy obviously was of non-zero thickness therefore what was in focus for slides would be out of focus for this and vice-versa therefore.
Show slides
Fold LCD over projector
adjust focus
Remove LCD thing
put slide on
adjust focus
You are, obviously, wondering why the slides weren't shown by PowerPoint. My recollection is that that was the rules. One of the many times I should have put my foot down but didn't. Sigh.