80 minute tapes
 

The other problem that plagues magnetic tape dating back to the open reel days is when thinner tape is used, magnetic print-through becomes possible. The magnetic charge can actually permeate from one layer to the next on the spool.

As has already been mentioned, tape stretch can also occur which is bad, very bad

If you have to use 80 minute tapes, try to make as few ff/rew ops as possible..