Crogar the Barbarian accidentally chopped his own leg for a lot of damage (something like 18+ injury) during our last game when he tried to cut down a giant fly that was on his chest. He just flat out tried to whack it, taking all sorts of penalties for being grappled (I think roughly -3 or -4 at the time), attacking into close combat (-2), and attacking in close combat with a Reach 1 weapon (-4). Oh, and a SM-1 target. He missed, and had a chance to hit the wrong target - in this case, himself.
He had a 9 or the original chance to hit, which was about the same. Either way, he rolled a 17.
I ruled it was a critical failure, he rolled on the table, got a 5, and down he went with a crippled leg.
I ruled that a critical failure - a 17 or an 18 - would take effect normally in such a case. My reasoning was simple, at the time: if it was a 3 or a 4 I'd count it as a critical hit because that's awesome. So therefore, a critical failure must be possible. Also Basic Set: Campaigns (see p. B392) called this 9 or less an attack roll, and attack rolls are attack rolls in my opinion.
One of my players argued this puts a PC who is potentially hitting himself in double jeopardy - don't roll a 9 or less or roll a 17 or 18. Seems unfair.
I totally see his point. But it's not the most common case. PCs trying to hit into close combat on something grabbing onto them happens infrequently, and swinging your axe at one isn't really a good idea at all. I can't see why that shouldn't have lots of potentially bad consequences.
Does this mean you can Critically Miss on a bow shot that is potentially Hitting the Wrong Target? Yes, but it's effectively meaningless to the shooter. So I don't see why I'd have to check for a broken string or a dropped bow just because I'd do so for a melee weapon. It's not the only way melee and missile combat aren't the same.
So did I make the right call? Maybe. But I'll stand by it and I'll continue to play it that way. I think it fits the game I'm running and its style.