I made the (train) ticket controller* brain hurt last time. While getting out of station C my ticket wouldn't work, and we have those doors that only open with a valid ticket, no way to pass above or under, and it's on another level as the counter or anyone that could help (stupid set up).
I was pondering going behind someone, which would be considered fraud even with a valid ticket if you get a grumpy controller.
Thankfully there was a pair of controllers so I told one of them that my ticket didn't work, and we had a few minutes of miscomunication, he thought I couldn't get in (even tho I was already 'in') while I couldn't get 'out'. Mix that with the fact that I explained it had been validated at 2 train station (perfectly normal and ok) that where 1h from station C...
*(I'm sorry, couldn't wikipedia the correct word, it's the person that checks the validity of your ticket inside trains/metro or even stations)