How much damage will manually focusing a camera lens when its on autofocus do to the lens?

Not that i make a habit of it, but its happend accidentally in the past... will this damage the accuracy of the autofocus?

Thanks for the answers so far. Yes its a USM lens.. glad to hear I haven't damaged it. I was wondering why it was so easy to turn the focus ring when my other non USM lens feel like they have a lock on it during auto mode. It's a Canon 50mm 1.4 lens btw. so so sweet.

It depends on what kind of lens it is. If it's a SLR lens that uses a hypersonic motor - such as a Nikon AF-S lens - no damage will occur. However, I wouldn't recommend fighting with the autofocus motor by turning the lens one way while it's trying to autofocus in the other. That would likely cause damage.

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If you are using a lens with an "ultra sonic" in lens motor, probably no damage will result.Some of the USM lenses are bult to allow touching up the focus after the auto focus has finished.

If you are using a lens with the motor in the camera, you might even damage the gears in the camera, and that would be an expensive error.