Bear in mind that I know nada about Minecraft or its mods beyond that it's written in Java, but here's a guess based on past experience with something similar.
An important difference between windows on one side and most unix and unix-like OSes is that the former doesn't give a damn about capitalization -- to windows, DoSomethingOrOther.java would be the same file as dosomethingorother.java, and a call to load the latter will settle for the former.
In MacOSX or linux or *BSD, however, DoSomething.java and dosomething.java are two entirely different filenames, and if the program is ordered to call DoSomething.java while only dosomething.java exists, it will fail.
(Your trivia for today: The second expansion pack to the original Neverwinter Nights, Hordes of the Underdark, had a similar error in one of its scripts that made a certain puzzle in the late part of the game unsolvable -- the script called for a file that wasn't capitalized as it should have been, which resulted in that animation not playing and a door not opening. Good times, good times...)