A few days ago the front mech started feeling really tight on the upshift, like I had to push past an obstruction before it would shift up. I checked the mech itself, the chainring teeth, and of course the cable route.
Nothing externally was wrong, but the problem remained. This was in the middle of a ride, so I decided to get myself home before I started the proper fiddling.
On the way back, I tried to keep front shifting to a minimum, but there were times when I had to. The last time I shifted, the obstruction suddenly kind of 'popped' somewhere between the shifter and the point where the cable first comes out of the outer on the downtube cable boss.
'Uh-Oh!' thought I, and rode home without shifting any more on the front.
Got home, undid the FD bolt, and started to pull the shifter cable out of the shifter. It was a bit jammed, but came out with a bit of fiddling.
Half of the cable strands had snapped in the same kind of area as the rear shifter cable had done previously.
Once again, I checked the outer, cable route, inside the shifter for anything dodgy lurking, the ends of the outer for jagged edges, etc.
I am waiting for Shimano to come forward and tell us this is a 'feature'.
Put a new cable in after checking everything again, thoroughly, and shifting is back to normal.
i have fraying of the PTFE coating close to the RD where the bend is tight and the cable coming out from the RD to the cable bolt. i figure with the fraying of the coating...eventually, there may be failure further down the road. i have not taken my cables out to inspect any wear where you suggest (20mm from the shifters), so i can't comment on that. i do have some binding issues with my rear brake cables (which i changed already). i'm guessing that the binding may be occurring where the housing exits the shifters because of a tight bend on my Enve handlebars. the housing groves on my handlebars don't line up as smoothly as everything else.