If a guard has 'incorrectly' endorsed a ticket I doubt whether pointing that out will achieve much.
I always take the view that if an authorised person wishes to stamp, write or scribble on my ticket then it is no concern of mine how they do it and I would certainly not be suggesting that they do it in a different way.
Normally I would agree with you and I wouldn't care less how it is done so long as it is done. Unfortunately real word experience has taught me that sometimes in these situations one guard's stamp/writing/scribble is not good enough for the small number of other staff who adopt the default position that anything out of the ordinary must mean you're a fare dodger.
You're under no obligation to get your ticket endorsed whatsoever, but if you don't then board the next available service (as you're supposed to do) you're then accused by some of fare evasion and lying. You learn your lesson and next time it happens get a helpful member of staff to endorse your ticket to explain you've been delayed to verify your story. It seems like the prudent thing to do, but this is interpreted by another member of staff as defacing your tickets and committing fraud. You just can't win :roll:
I did suggest on another thread a while back that perhaps some sort of industry-standard service disruption or permission to travel vouchers should be adopted. I think someone said such a thing exists but the industry is incapable (or perhaps unwilling) to implement it.