Today I experienced a weird bug with one of my 600EX RT (I have two).
So I put on the included gel holder and then try turning on the flashes, one of them turned on just fine, the other one shows low battery and could not be turned on. Fine, I will change the battery. However, after switching a couple set of batteries I found that it always says low battery.
I almost started to looking into sending it in to repair. But then I took the gel holder off the flash tube and then it turned on just fine. Then I put on the gel holder, turned it off and on again, works just fine.
Has any of you experienced this phenomenon?