Took some shots with the kit lens, then some macro. The light was fading, so I pulled out my spare sun - the SB800. Popped it on the camera (both SB800 and camera were off) and it turned itself on, then off.
Then I couldn't get it to turn on again. On camera or off, with a new set of batteries - no action. The sun was dead, I thought.
Had a closer look and found this on the 5th battery compartment:

I tried brushing the crud off, then scraping. Eventually, I managed to get it going again. Once it turned on - no problem. I think it may have been a dud battery? There was a bit of crud on the corresponding battery, but it didn't look like it was leaking or bulging.
Anyone else had this happen to them?