A friend of mine decided that the heads in his VTR needed to be demagnetized. As his arm came back and away from the heads with the demagnetizer, it came near his tape library. The end result was that several tapes were ruined. Because of costs, the tape cassette isn't as protective as it could be. It does a fair job of protecting the tape inside, but not a complete one.
Dust can still get inside, for example, as can any other airborn contaminant.
The solution is easy. When the tape isn't in actual use, remove it from the VTR and put it back into its jacket. Better yet, put it in the jacket and then into a tape storage box. As long as the tape is in the machine, the protective front flap is open and the tape is exposed. Removing the tape closes this flap.