In REAL life, the range of possibilities is pretty wide, but here's my take.
The maximum penalty is 5 years in prison and $10,000 fine. But the chances of actually receiving that sentence is nearly zero, unless you somehow personally piss of the judge in your trial.
In MOST cases where you were found to be in possession of restricted mags, they'd likely just be confiscated (read: stolen) "for the protection of society". Don't be surprised if your (otherwise LEGAL) rifle is also confiscated, though with work, you could probably get that back. Don't expect it to be in good condition, though...
A realistic "worst-case" is that BATF is contacted, and for whatever reason, felt that they had time to bother with such a trivial violation (most of the time, they literally will inform the cops that they don't have time to deal with petty crap like post-ban mags). Then, you could be pretty screwed. Most likely, you're the target of some person who is looking to make a public example of you, and the best you're gonna do is plea bargain the charges down to something more managable (misdemeanor with fine and/or community service).
I agree that in most cases, no one is going to care in the first place. Only a hard-core anti-gun LEO is going to make it an issue in the first place.
-Troy