It's a full run of cable housing so there's only two brake ferrules (one at each end of the housing) and they were both seated fully, with the housing fully seated into them.
It's worrying to think this could have happened out riding, albeit this was on a rear brake. I've had this happen in the past setting up BB7 brakes and still don't know what causes it! I pulled about another 15mm to 20mm of cable through the caliper and tightened it again, that has presumably fixed it.
What causes this? Is it the brake lever end of the cable snapping into place? I don't think it was that because it doesn't explain the cable going slack by 15mm.
What's 15mm? A ferrule...
Maybe what happened is, the brakes were slack when setting up and a ferrule ended up resting on the lip of the rear caliper ferrule hole - you wouldn't see this, because it's got a rubber boot on it. The same effect would happen if the housing came out of the ferrule and the ferrule stayed in the caliper, then it would be the housing resting on the lip, but the exact same end result would happen.
Pulling the brake several times re-seated it. This is the only thing I can think happened but I wondered if it's happened to anyone else or is a common "schoolboy error"
