Staples case and malfunctioning semiautos
Snowflakes in Hell has a post on the issue of whether a malfunctioning semiauto can leave the owner guilty of an NFA violation if it doubles.
I'd agree with his assessment that the Staples case largely settles the issue. The National Firearms Act defines "firearm" to be an NFA arm, not any firearm (Congress can define things any way it wants). The question in Staples was a split in lower courts. They agreed that the defendant must be proven to know that he had a firearm, but split on whether that meant the NFA definition or just anything that went bang. Supreme Court said the NFA definition applies.
UPDATE: it was, I think, a core issue at Olofson's trial. Problem from defense side was that the prosecution could call the guy he loaned the gun to (to test before buying), and that guy (true or false) claimed that Olofson told him not to put the selector switch in a certain position (in which the gun began firing full auto). From this the government could argue that he knew the gun was malfunctioning and had so for sufficient time without getting it repaired. That's why I say, if it happens, go straight to a gunsmith.
So if a semi doubles by surprise, the possessor didn't have the required knowledge. Now, this might leave open the question of whether he violates the law by continuing to possess after he knows it will do so. So it's clearly time for a quick rush to the gunsmith for repair.