I hate when this happens. Couldn't see it till after stained. Had to start over. Bummer. One of the downsides to a heavy duty belt sander. (Upper left corner)
2nd attempt came out much better. Just lost about half a days shop time.
Just say the kids were running their fire truck on the table
Yeah, belt sanders are a real double edged sword tool. Sometimes, they are awesome and the only thing that will work. Other times,..... well they cause more work fixing their "Fix"
A half dull sanding belt will do that rather quickly. Also a belt sander that isn't balanced over the center of the platen will rock and dig in on one side.
Old photos below, but it gets the point across.
The Porter Cable 360 series of sanders balance well for a mid-priced sander.
Porter Cable model 500
Unknown Asian-made sander. Its pretty obvious by the front roller that something was off.