True Colour GIFs(Potentially relevant link)
My Intro CS textbook--Computer Science: An Overview (6th ed.1) by J. Glenn Brookshear--described, among other things, the GIF image format. According to it, GIF compression consists entirely of limiting images to 256 colours (as opposed to BMPs which aren't compressed). Of course, as someone who wasn't completely clueless, I knew essentially how GIF compression worked and that BMPs can use 8-bit colours, or even 2-bit colours.2
What I didn't know until today was that GIFs can in fact be more than 256 colours. That's amazing. (Note for people like me who have animations deanimated: the true colour GIF at the above link is an animation.)
1 I think 7th ed. was recommended, but those of us who'd bought the used copies had 6th ed. and that was fine too--so long as we noted this one error in this one table we used. I later got a 4th ed.
2 My professor, of course, was fairly clueless. He was in IT, and while he may be a good IT person, his C code would contain lovely keywords like 'If' and 'While', he told us i/5 + i%5 would give us i/5.0, and probably other nutballish things. A nice guy, though, and I believe fairly competent at his job when it doesn't consist of teaching courses outside his area.