At the REI garage sale a few weeks ago I got a Fenix E12 for $11, in perfect condition. I like it, but the output pales some in comparison to my D25A and SC52 on 14500. According to what I can find by searching online, a 14500 isn’t supported. I’m tempted to try one anyway, I just don’t want to kill the light.
So what do you guys say, try a 14500 and cross my fingers, or stay safe with NIMH AAs? What’s the chance that it’ll go poof?
I have the previous model, Fenix E11, it do not work with 14500.
Stay safe 
Wait a second. I can’t believe you did not share that deal with your fellow BLFers! 
Did it go “poof” or just not work at all?
Oh, haha… the garage sale is a sale of returned items. You have to be there in person and buy it on the spot. I was just lucky. there was a Nitecore SRT? light too that I saw but didn’t grab. Still kicking myself for that one.
I wouldn’t recommend it. I have 9 Fenix lights (in the PD, TK, E, and HP series - all, great lights), and the written instructions are very specific about what batteries they can use - for a reason.
Like the other poster, I have an E11. I looked up your E12 and found the PDF manual for it. They also specify an alkaline or NiMh battery for it - so, 1.2 - 1.5 volts, on average.
You could try using a 14500 but you’ll probably burn up your light. You’d essentially be inserting a battery that produces nearly three times the voltage that Fenix recommends you use in their light. I don’t think it would end well for you. If it worked at all, you’d probably get an incredible amount of light out of it, but not for long.
You’d also void the warranty, and you’d be stuck with a very pretty paperweight, rather than a nice flashlight 