Imagine I had an Oyster card with both a Zone 1-2 Travelcard and a
Zone 5-6 Travelcard loaded on it -- not a completely implausible
scenario, e.g. I might have an annual Zone 1-2 Travelcard but the need
to spend a week in the outer zones so add a weekly Zone 5-6
Travelcard, too.

Imagine I then took a journey from Zone 1 to Zone 6. Would the system
charge me the correct fare for the journey? More to the point, would
the system charge me for the journey at all?

-roy