I decided to add a memory to my GM script. I added a single call to GM_setValue and suddenly script stopped to be called at all. I tried adding @grant line to the metadata block, tried reinstalling and renaming the script - no luck.
Moreover, commenting the lines with GM_* functions doesn't help. They must be deleted in order to script become functional again. Even mentioning them in @grant line without actually calling prevents script from starting.
I'm experiencing this on Ubuntu-12.04, Firefox-19.0.2 and GreaseMonkey-1.8