I had an issue with thumbnails generated while option "thumbfix" is set. Some images did not apply to the desired thumbnail format (e.g. 150x100) but where significantly smaller (133x100).
The issue was fixed with a patch from post http://wordpress.org/support/topic/plugin-nextgen-gallery-crop-thumbnails-1?replies=10#post-1276732
I'd like to see this fix integrated into the next version.