The first Collett designed 5600 0-6-2T was steamed in 1924. At first movement part of the valve gear promptly bent out of shape as the draughtsmen has overlooked giving it some support. Collett is reported to have told everyone present "This did not happen!" and it was hushed up for years.
Some real doubt as to whether that incident actually happened. It is quoted in Gibson but disputed elsewhere.
On LHCS, the introduction of the Mk2d stock wasn't without incident. We lost a few out the door with those very short lived interior door handles. The auto announcements (a big tape machine!) didn't really work and soon went as well.
The start up on the MA sets used to knock hell out of the static ETH, the Deltics & first 20 47s so the coaches had to have a time delay relay fitted, one setting for each type so when the power went on it didn't trip out.
Mk3 stock. Where to start? Bog door locks that were over designed so that vibration would displace internal springs and the door would fail safe, shut and trap the punter inside. Kitchen cars. Microwave oven doors that would pit and then hole around the handle, no proper securing of cupboard equipment so the fat bucket would fall out on the Offord curves making a mess of the car. The beer dispenser with the oh so handy in line test cock!
Underneath the wsp on the big kitchen cars which spalled the wheels within weeks of entering service. Buckeye couplers that were a pig to secure on curved roads in depots, leading to buckeyes parting in service and sets running as effectively two trains. You used to detect it inside by looking at the buffing plates - a big jubilee clip round the gravity block was the initial emergency fix on that one! Not ensuring that the double glazed windows were put together right so that one day a window broke, passed another at speed, scored the glass there, train passed another and the windows broke, and so we went on. 120 windows in one day on the ECML. The lorries were busy that night getting the new windows to the depots.
Spanner 2 boilers on Deltics. Could not heat much beyond 9 coaches. Way inferior to a Spanner 1 or 3. The KV control modules on the cl.50. Normally lack of volts means lack of power but some bozo at EE thought it the other way round. Ever seen a cl.50 "lock on" to full power when just sitting there? Fearsome sight.
Even wagons. COV AB vans that would run away because someone couldn't design the brakes right. The private diesel shunter at Marshmoor was forever having to rescue the Kellogg's traffic vans from the bottom of the sidings and one poor shunter at the MOD Bicester rode on his brake stick through a set of roller shutter doors thanks to one running away. Big enquiry after that one.
Just a few. M&EE on BR was a great place to learn about mistakes!