I inserted a USB key with a FAT32 file-system on its one and only partition and mounted it on a newly created directory in /tmp. After mounting, I did an ls to verify that what I expected to be there was there, and it was. Among the contents was a .iso file that needed to be copied to another USB key. I inserted that key and began typing the dd command to do the copy, while cd-ed to the directory where I'd mounted the first key. When I hit 'tab' to complete the name of the .iso file, it would not complete. So I backed out, did another ls and got nothing! No files! But the file-system was still mounted, confirmed with df. I suspect that the insertion of the second USB key confused the system somehow and it lost track of the first.
I should mention that I did try to reproduce this and have not been successful thus far. The key in question is a Sandisk Cruzer Facet, 8GB
The second key, the key to which I wanted to do the copy, was a Kingston DTSE9, 8GB.
Unsure about where the error lies, but assume that it's in msdosfs.