Overdistance
Here's an example:
Buying a ticket
Orpine wishes to buy a ticket that is not available from his origin, so he either has to buy on-board (which Chiltern don't want him to do) or buy the wrong ticket, and get an excess to the correct ticket. In that case the excess is zero as the fares are the same.
Change of route
See
4.2.1 Excess procedures: change of route In one case a passenger held a Winchester - Lancaster
SVR via: Bannbury. But they wanted to travel via London, also at a time the
SVR via: London was valid. Train Company staff could say "well the ticket held costs more, so no need for an excess", and indeed the first guard held that view. But problems arose when the passenger then tried to use LU, and further problems arose when the passenger encountered knowledgeable staff at the Euston gateline who were not trained in excess fare procedures and claimed a new ticket was needed. :roll: A zero fare excess issued by the first guard would have avoided those problems.