One Sunday morning, when travelling from Cardiff to Crewe, I got rather panicky when we didn't turn left at Maindee, but carried on ahead toward the Severn Tunnel.
Having glanced around and checked that yes, I
was on a 175, and hadn't accidentally boarded a FGW train, we came to a stop. A few moments later the driver walked through the train, and we reversed along the Maindee curve toward Shrewsbury.
As it turns out, this was NOT a wrong route taken (to the best of my knowledge), because we were right time at Cwmbrân and onwards. It seems that it was a planned diversion for route knowledge purpose, but nobody had thought to tell us passengers