I just noticed this earlier today.  I have copying a relatively large video file from my camera to my computer, with both the source and destination directories open.

The copy took about 3 minutes, and during that time thumbnails of the destination video file kept getting regenerated as the file changed.  However as Thunar knew that it was writing that particular file I think it should have refrained fromm thumbnailing it until it was finished.

For all thumbnails, I would suggest:
* start writing file,
* at 10% run the thumbnailer on that file (my reasoning being that videos, pdf etc can be thumbnailed at 10%)
disable thumbnailer until the write operation has completed
* re-thumbnail the finished file

I've marked this as major because, even though it's not a show-stopper, it causes a fair amount of disk thrashing and wasted CPU time.