To "forge" bitcoins would mean to get an unauthorized (by me) transaction from that address.
I'd call that "theft", not "counterfeit".
One thing to worry about is forging (replacing) public addresses with fake ones.
Another form of "theft", still not "counterfeit".
Which again would not permit counterfeiting, only theft.
But there may be bugs in the dominate client sofeware, like last time's overflow error.
That's the best suggestion I've heard so far.
You're right, certainly there could. And if one such bug were to appear and be leveraged, it would be repaired, just like it was last time. So I suppose
if there is such a bug, and someone can figure out how to exploit it, the coins that would be temporarily created could be considered "counterfeit". Especially if the "counterfeiter" can get anyone to accept them. Then after the repair, the coins would cease to exist, and the result would be "theft".