A 'wrong route' is an excess, not Penalty, fare, and that's the way it should remain!
Agreed.
It is not helped by the route-planners only showing the "quickest" route, and if staff are asked "How do I get to X?" they will often say the quickest/most common route, not "What ticket are you holding?".
So it is difficult to know if it was a deliberate mistake or confusion, but the passengers reaction when it is explained and the excess requested is usually a give-away.
Sometimes people can be sent back onto their "correct" route, and are happy to do that.
Some people are very happy to pay the excess and apologetic, some people are VERY upset and aggressive to be asked to pay and it sometimes ends in tears (theirs).
The change-of-route excess can be anything from 50p to £20-£30 depending on routes, tickets and time of day.
Personally, I am more inclined to charge an excess if the passenger is changing trains, especially on to another TOC's trains, as I figure it's not fair on the guard on that train to send them my problem, especially with a stamped ticket and the ready-made excuse "The guard on the last train said nothing".
If they refuse to pay, I stamp the ticket, endorse the ticket "RTP excess" and give it back to them, and depending on the station they are changing at, I'll inform Revenue at that station and pass it on to them.