Error 99 Changed F-stop! Have You Had This One?

I was photographing in my back garden today (20D and 70-200mm f/4 L) and I got an Error 99. I switched the camera off, then on again and it was fine. I carried on photographing only to notice shortly afterwards that the f-stop had changed from the f/5.6 I was using to f/4.0! I altered it back to f/5.6 again and carried on with no further problems.

I haven't had an error 99 before! Have any of you experienced that particular occurrence? Do these things happen occasionally anyway, or should I be concerned about it?

Thanks

Mike

What mode were you shooting in? Tv? Av? M? P?

I was in Manual mode.

Since you were in Manual, you probably accidentaly changed aperture yourself, while you were reseting the camera.

I thought that initially, but having seen how many of clicks of the adjuster wheel it took to put it back I realized I couldn't have done that. When I looked back at the photographs the first photo showing as f/4.0 was the very first one after I got the Error 99.

Mike

Between f/4 and f/5.6 are 3 clicks.
Not so difficult to turn accidentally, especially when you are excited because of the error.
Wait and see if it happens again, nothing else you can do at the moment.

I would have needed to dial it in with the end of my nose to change it whilst photographing and I certainly didn't do that!

I'm not too worried about it, rather that I'm curious to know if anyone else has had it happen.

Mike