Help with Eclipse - Apparently it has a mind of it's own?

OK, so I have EGit set up to pull Bukkit directly into Eclipse, and M2Eclipse to handle the Maven setup. Yesterday, this was all fine and dandy, and tonight it was as well; except suddenly, and without me touching it, it just started throwing errors.

At first, it was 10 errors saying it couldn't find java.lang.Enum and to check my build path. Then, again without warning or me changing anything (just opening up random files to see what it was looking at) it changed to about 2477 error saying that it can't resolve any types that Bukkit defines (Achievement, Action, Block, etc)

I could understand this if I was messing about with stuff, but I haven't touched a thing. I have edited my own projects but that's it.

Has anyone had this happen before?

it's Eclipse Helios with SVN, EGit, and M2Eclipse installed. vanilla setup, not doing anything crazy. Sun Java 6.

UPDATE: I'm deleting and re-adding all the Workspace projects. As I re-import them, they seem to be fine.

Here is another oddity: Maven would build everything just fine (before I wiped it out), Eclipse would scream that nothing would work. Is that normal?