Long story short, my 550ex was stolen at a wedding and I was given a broken 580ex to have, I just had to get it fixed. Sent to Canon, fixed (took forever), got it back. Went out to shoot and it started giving me problems. Canon took it back to repair, fixed it, sent it back. Took it out to a wedding and 25 mins in it started acting weird.
Symptoms:
- Flash will not turn on
- Zoom motor will keeping zooming back and forth
- Flash will not fire
- Light clicking noise coming from the flash after it is turned off
- Turn it on-off-on again and it won't turn on
- Batteries extremely hot
I have called Canon to ask for a refund on the repair, they said sure but they want it back to test if it doesn't work first. I said fine. So in the mean time, I went and test the flash again and I've narrowed it down to one set of bad ni-mh batteries that are 4 years old.
Batteries:
- 8x Panasonic R2 2050 mAh (2 years old)
- 4x Panasonic 2300 mAh (4 years old)
- 4x Panasonic 2450 mAh (2 DAYS old)
The above batteries work fine in my brand new Canon 580ex II (from B&H 2 days old) no problems. Put them in the repaired Canon 580ex, all work fine except the 4 year old Panasonics. They will work when u first put them in. But turn it off and on again and they don't work. I think this set will also give me all the weird symptoms as before. I'm currently charging a bran new set of Sanyo 2500 mAh batteries and will test with the two flashes again. If the repaired Canon works fine then it's definitely the old set of batteries and I'll keep the repaired flash. I'll update this after they are charged.
Conclusion:
If you are getting any of the above symptoms with your flash it may be because you are using:
- Using a bad set of batteries
- Using a set of batteries with a bad cell
- Using a set of batteries that are toast
- Your flash is actually screwed and you need to send it to Canon. Just note it will take about a month to get it back.
I would suggest using a fresh set of Alkaline batteries as well. Those worked fine for me.
PS. Thanks Justin for the flash

. w/o it I wouldn't have found out all of the above. It would seem that others online have also had similar problems.
Edit:
Flash is still giving me erratic problems. LCD will flash when turned on. It will eat batteries if you leave them in it as well. It's going back to Canon for a refund.