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If you have enabled debug mode for resourses, in Config.groovy:
grails.resources.debug = true
plugin doesn't apply, and you are getting source of your .coffee file.
Actually it's because of resource plugin (see ProcessingFilter:32 for v1.1.6), but it's really not what you're expecting by enabling debug mode. But maybe you have idea how to deal with it?
add note about problems with debug mode, see #15
FYI this issue is being tracked here: http://jira.grails.org/browse/GPRESOURCES-180