While I'm sure this can't be the first time someone has discovered this, I didn't see anything specifically related to doing this in any threads, so I thought I'd just drop a comment here about it.
So I had a bowyer strange mood, and of course he was trying to make a wooden bow/crossbow/blowgun. I wanted to make it out of highwood, so I forbid all but highwood before he started, but he wouldn't take any wood. He wanted a specific type, in this case tower-cap.
So, being the sometimes spiteful overseer I am, I let him gather his materials, which consisted of two tower-cap logs, some dolomite, and rough and cut gems, and start work on his creation. As soon as he started, I hit "t" and looked at the materials he was working on in the workshop, and promptly forbid the two tower-cap logs. "Make your crossbow now, mr. tower-cap user!!"
I figured he'd just go insane, and that the job would not complete at all. Or at least, something along those lines. But, loe and behold, Urist McTower-cap did indeed finish his creation. Except, we ended up with an iron bow.
Iron, while it is in my fort, wasn't even one of the materials he gathered for the job. I would have thought that, if he didn't go insane and did indeed complete the job, that at the least it would have used one of the other materials he gathered for the job (even if not normally available for whatever craft being made, such as a bow).
This has made me very curious. Thanks to this guy, the next several strange moods I get, those poor dwarves are going to have the main material for their item forbidden by me, after they begin construction, in the name of Science. I would like to see if it just chooses a random appropriate material when the strange mooder loses the main material once the job has started, or whether its always iron by default, etc.