Two Rights Make a Wrong
We can't get gas or grenades from the black market, not even knowing that it exists, but that's not stopping Harry Herpderp, not his real surname, from arming himself.
'I've bought a perfectly legal shotgun, and a perfectly legal hacksaw', he says, before taking one to the other.
'Do you know what you're doing?' At first blush it looks like he does, ending up not with a shot-up hacksaw but a sawn-off shotgun.
'I could make a mechanical repair check to see if I do a good job.' The GM agrees, and Herpderp makes the roll. He fails. 'How did I do?'
'I imagine you'll find out when you decide to use it.'