Strange frame on K10D

[fishy]

Last weekend I was out shooting and after swapping lenses (replaced the Sigma 30-700 with DA* 16-50) I had a few shots (maybe 10) that had a weird rotated border.. Looks like something loose inside the camera. I checked, but all looked normal.
Should I be worried? Never seen this before (or since) shot of cloudy sky only:

[Ash]

Was a lens attached when this photo was taken?
It looks like an actuation was made without a lens on here, but I'd need to know more...
Did you notice the shutter going off while switching lenses?
Do photos come out fine now?

[fishy]

There was a lens. (DA*16-50) This problem was there for a few shots. When I noticed, I took off the lens and checked the sensor (mirror-up) could see nothing wrong and problem gone.
I have a few 'normal' pics with this phenomenon, I just took this one as a test shot, to make sure nothing was hanging in front of the lens. Yes, photos are fine now.

[Zav]

The last time I had sthg like was when my lens hood was not correctly mounted.

[fishy]

Hey! Good call! (Feel a bit stupid now...)
Can sort-of reproduce it, just not with such straight edges. but I'll be more than happy to accept it was the hood (and me) and not a sensor issue...