Fortunately for you I have the answer! Through direct experience tonight. A couple of pops, some hissing, and a wiff of acrid smoke and voila: a fried recharger. I think the batteries are spoiled as well... though hopefully not the other two REAL rechargeables that were in the same charger at the time.
Note: don't try this at home. It's probably not that dangerous, but it sucks to ruin a perfectly good device. Now I have to go and buy a new charger. The smell is pretty bad as well.