There can be some add-on sources of vignetting (as Don said, especially when the lens is at its widest)...
They can include things like a matte box... or stacking on too many screw-on filters so that they stick out too far.
One of the challenges is that viewfinders often overscan a bit and you can't see the problems until later.
It's funny, you often see a lot of "clipped corners" video on television commercials. You won't notice it on a regular tubed television (CRT) because of the slight overscan... but they show up when viewed full-frame on an LCD or plasma television or on a computer monitor (or on a CRT that underscans).
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