I had a problem on a bike with internal cables. I couldn't understand why I couldn't get the brake to adjust. No matter what, it was too loose. Turns out the bushing broke in the internal cable stop and the plastic cable housing was acting as the housing, not the internal metal sheath. So as the tension increased, the plastic would compress. Took a bit of time to find that one.

I've also had the problem described by lhbernheardt. The cable started to break, one strand at a time. I kept having to make adjustments for the FD. Finally it snapped completely. THEN I knew what was causing the stretch...