After rearranging many clips on the timeline (by either dragging or cutting and pasting), I noticed that some clips, although having the correct filename and duration, had the content copied from other clips. A short clip occupying the site of a long clip was repeated, as happens if you drag the end to make it longer.
Could I have done something wrong to make this happen?
I repaired the problem by deleting the scrambled clips and re-importing the correct ones. I hope nothing else got scrambled that I haven't noticed.
I don't feel inclined to mess about randomly shuffling clips ad nauseum to see if I can reproduce the problem.
Windows XP SP2 (32bit) and Vegas 9.0c (32bit).