I only saw the Bilingual positive quality as stating it "can only be acquired at character generation."

Does that mean that all others (except for those that cannot be taken under some circumstances) are fair game for their listed Karma cost x 2? In what cases do some gamemasters rule that a positive quality must be purchased at chargen?

I'd generally require a reasonable explanation of some sort, but as long as there is one I can't think of anything I'd rule out.

The explanation can be something like "I had this all along, but didn't know it." For example, "You know, I just discovered when we got shot up that I heal quicker than my teammates." Or, "You know, at the firing range I thought I'd try shooting left handed. It's easier than most people make it out to be."

Works, if healing time or offhand penalties hadn't previously been an issue. For stuff like that, though, it's very possible for me to create an excuse for a player's character to pick up the quality.

I had complex and risky surgery to replace all my blood and bone marrow for a type O system.