Your idea relating to double booking reminded me of a blog post I wrote a while ago about testing a flight upgrade situation (my mindmup is here: https://danashby04.files.wordpress.com/2015/04/real-istqb-test-question.jpg)
Like you, I’d probably collect data about overbooking scenarios (I recently had bought a flight ticket and was told I couldn’t check in online and when I got to the desk at the airport, they informed me that I didn’t have a seat on the plane).
On top of this, I’d look at data. Data relating to: number of seats available (across classes), number of tickets booked (in each class), type of plane, destination and flight frequency to that destination and flight duration to that destination (for “workaround” options to be able to circumvent risks from that angle).
I would then look at the check in system itself - the different methods of checking in (online, self service kiosk, in person at desk). I’d look at the performance of the systems (data transation delays and error handling). Localisation is a big risk (passenger data, such as their names if they contain any special characters… Accented characters, foreign characters, Double barrelled names with a dash, or a single quote (O’Malley) could be problematic)
I’d look at risks surrounding the upgrade (or downgrade) options through the system.
I’d also look at the timescale restrictions relating to “allowed” check in hours and restrictions, and play with the rules.
There might also potentially be escalation routes within the system for handling an overbooked flight (it’s common for flight companies to do this, so I’d be surprised if the check in system doesn’t support that). Perhaps it’s help text and contact info, or perhaps it’s a resolution feature built in.
Then I would look at accessibility risks. Can someone request accessibility assistance when checking in?
And there is so many things unmentioned that relate to checking in. E.g. baggage… Multi traveller (sitting together), travelling with babies or children, being given an emergency exit seat (what about the rules for the emergency exit seats? E.g. pregnancy, fit and able, age restrictions, etc)…
It’s certainly turning out to be an extremely complex system…