Fr33m4: I've finally been able to experience what you've been talking about. There's this ROM here called 'Mystic Quest Legend (G).sfc', it shows up in green, and when you click on it, it hangs the emulator. Yep, renaming it to .smc does the trick.
The impression I get here is that these ROMs that show up in green use a foreign extension but got renamed to SFC, and for whatever reason, when you feed this to Memory.LoadROM, it crashes the emulator.
Very weird. It also seems to be mostly European-localized versions showing up in green (France, Germany, and so on).
What's the file extension of that ROM, btw? Does it end with .fig, .sfc, or .smc? The way the rom_menu function currently works, it won't recognize any other file extension other than those three.