It was on 1/16 (lowest) power being used as a fill for my 580EX. Was firing 2 pops at a time during a formal shoot before a wedding. Just a malfunction/poor QC. The thing is probably 20+ years old though. I'm not terribly upset, just gives me an excuse to get another 580.
During the shoot I noticed it wasn't firing as often as it should have been, especially with a fresh set of batteries. I went over to check on it and I could smell the plastic burning. It was super hot to the touch. I just said screw it and continued the shoot with 1 flash and upped my ISO to compensate and get additional ambient fill.
Irritating, but I didn't burn down the church, and the "after" photos look great.