It looks like we can't define functions named true(), false() or null(). Actually we can define them, but they respectively return true, false, null, no matter what they are supposed to do.
Test case: http://sassmeister.com/gist/8595565
The problem doesn't seem to occurre with LibSass though.
By the way I also tried other keywords like function(), mixin() but that doesn't seem to have any incident.