The CoffeeScript (compiler) sources are full of trailing whitespace, so trying to edit them with a non-customized coffee-mode results in a mess.
In general, this seems like a feature that can be useful, but has no place in a language mode.
In fact, this feature is duplicating whitespace mode. I've added a pull request to remove it: #78
Remove support for removing trailing whitespace: whitepace-mode does …