retroproyector
"Proyector" is "Projector" in Spanish.
The overhead projector was invented in the 1960's.
The projector can reach a pretty high temperature if left on for a while. So I think the main danger of an overhead projector is simply the heat. ;)
My guess is that with LCD and DLP projector coming down in price, we will be seeing less and less of the overhead. When a video projector is used with an interactive whiteboard or tablet the possibilities are limitless.
projectionist
You don't need a slide projector or an overhead projector.
This is referred to as an opaque overhead projector, or sometimes simply as an "opaque projector." This is in contrast to the regular overhead projector which must have transparencies to project, an opaque projector can project an image of opaque objects such as the paper pages of a book.
Using projector can look neat and professional. Masking way can be used to hide the content that you haven't say them. Then you can reveal one piece of information at a time when you talk about it.
The only real similarity that I can think of is they both project images onto a wall or projector screen.
Electricity
Overhead projector replacement bulbs will definitely be sold somewhere on eBay or alternatively you could ask the local secondary school or college for help.
One answer is when an overhead/movie/computer projector does not fit the projector screen area correctly and lines can be seen around the intended area.