Errors while using rox filer to dump home directory to usb

I was helping a friend to dump her home directory using rox filer, which creates a separate window to list the files going across. Towards the end of the dump, she drew my attention to a reddish flickering in this window. The files involved were tiny so they were moving very fast, but it looked to me as if each file was being displayed both in black and in red. At the end, rox reported over 1000 errors!

The files in question were all in ~/.cache/mozilla so I don't believe any of them was of long-term importance, but I am still curious to know what might have caused the errors. I'm not very familiar with rox.

I don't think we could have run out of space because there was only 4.8 GB to copy altogether and the memory stick was bigger than that (16 GB afair).