Zone Of Truth is not a cage.
From the source:
You create a magical zone that guards against deception in a 15-foot-radius Sphere centered on a point of your choice within range. Until the spell ends, a creature that enters the spell’s area for the first time on a turn or starts its turn there must make a Charisma saving throw. On a failed save, a creature can’t speak a deliberate lie while in the radius. You know whether each creature succeeds or fails on its saving throw. An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such creatures can be evasive in its answers as long as it remains within the boundaries of the truth.
This description and its implied rules are far more concerned with handling the definition and limits of the truthspeaking aspect of the spell. What it doesn’t state (and is almost certainly a deliberate choice) is whether or not the affected target needs to stay within the bounds of the spell. I’d argue they definitely don’t.
Unless otherwise constrained (say under the influence of Hold Person, or grappled, or held at knifepoint or you find yourself inside a forcecage) there’s literally no reason a PC or NPC can’t just walk (or run, or teleport, or climb or float or fly or swim) away from the zone. And yet I’ve never in all my time DMing seen any player realise they can do exactly that.
Obviously there might be sundry reasons (like those listed above, as well as good story reasons) why a PC or NPC can’t just flee, but psychologically most players seem to assume ZOT = Magical Inescapable Mobile Courtroom. But that’s simply not the case.
Just a weird loophole I’ve noticed.
What other weird loopholes have you noticed in D&D?