Subject being brake servo vacuum hoses. Not sure quite how universal this failure is, but I've seen so many now on friends and neighbours' cars I always check them any time I remember to whilst under the bonnet of a Volkswagen Group car.
The hard plastic sections split where they are stretched over the barbs of hose nipples on the terminations at e.g. the non-return valve.
If the hose completely parts company with the bit it's attached to you will lose all servo assistance for the brakes. The brakes still work, but you have to press a hell of a lot harder on the pedal. If you don't know this, you will believe the brakes have failed completely.
Before this catastrophic failure, if the splits develop to the point of leaking significant quantities of air, all sorts of weird fault codes can be thrown up, leading to replacement of non-faulty parts if diligent diagnostic techniques aren't used. Lambda sensors, throttle bodies, all sorts of things could get the blame due to 'false air' entering the intake manifold through these splits.
The splits I've just found were not super-obvious. The first one on the underside, I felt with a fingertip before bringing a mirror out to inspect it. The second split, and worse one was on the front side, facing the engine, and couldn't be seen or felt easily until I pulled the whole assembly out a little to inspect more carefully.
How old was this hose assembly on my car?
5 years only. The original was found in a similar or worse state about that long ago, this was a new genuine replacement then.
Guess what improvements have been made in materials and design of these hoses since this car was made in 2003? None whatsoever that I can see.
Please check your own car's equivalent. Go on, you have some time in the next few days, right?
If you find crap like I just did, please photograph it and add to this thread.