I was shooting in a corporate office space using a 580EX II as on-camera master with 580EX off-camera as slave. It was working fine for some initial shots. However, for some reason the off-camera speedlight locked up at full power. I could switch between ETTL and M, but was unable to lower the output. Pressing the center dial button did nothing. Tried powering off and on, but there was no change. Had to abandon the particular shot since there was little time and the employees were waiting. Odd thing, 20 minutes later on a different shot it was working fine again.
Wondering if the 580EX is malfunctioning or if there is some trick to getting it back to normal if this happens again. Any suggestions?