Event AEE not so A?

This is quite true. I believe Maple uses the same system for 100%s as the first generation Pokemon did, which is a rather outdated system.
In this system, "100%" is really 255/256, so you still have a 0.39% chance of failing it. Want more proof? Go watch any Pokemon TAS of the 1st gen games: 100% accuracy moves miss many times over.

It's the same thing with these 100% scrolls in Maple: they have a 0.39% chance of failure. Of course, this was in v0.55, I have no idea if they changed it when big bang came out.

I've never seen proof of a 100% failing. If it was 0.39% chance of failure there would be tons of videos - just like the Pokemon. It's not like it would cost a lot to buy ~1000 100%s and use them until one fails on video.

Maple Story is a computer game, there is absolutely zero reason for MS to use a crappy gameboy RNG that produces an 8 bit int - it would be harder to even find such a thing (because of the flaws like not being able to give 100%) than to use a proper thread-safe random function, periodically synchronized with the server. MS has plenty of "break points" where the game won't let you do anything until the server responds - healing, using pots, etc. - it would be pretty trivial to resynch by sending an extra couple bytes along with each of those packets.