Source: 8 minutes 56 seconds juncture, Lecture 12-2 (transcription), ... How to Reason and Argue,
by Prof Ram Neta PhD (U Pittsbugh; in Philosophy)
Okay, now refutation by parallel reasoning doesn't always work. Sometimes, we get results that are unclear or don't show what we were trying to show. [...] consider the following argument.
[Premise 1:] If guns are outlawed, only outlaws will have guns.
[Premise 2:] If only outlaws had guns, it would be bad.
[Caution: I reordered this premise; strangely, Neta preposed the apodosis before the protasis].
[Conclusion:] Therefore, guns should not be outlawed.
[...] that's an argument. Is it a fallacy or not? Well suppose we try to refute it by parallel reasoning ... [by replacing 'guNs' with 'guMs'.] Now notice, this parallel argument has exactly the same form as this earlier argument about guns. The two arguments have the same form. So if one of them is a fallacy then the other one is a fallacy. But is this argument a fallacy. That's not clear.
[...] So we can't tell by looking at the parallel argument whether the earlier argument about guns was a fallacy. So this is a case of refutation by parallel reasoning that doesn't succeed.
Where is the fallacy? The argument appears to lack only the following Suppressed Premise:
[Premise 3:] If it is bad that only outlaws had guns, then guns shouldn't be outlawed.