I came across a situation where one of my PCs were fighting against a caster that had readied booming blade for if they provoked an opportunity attack.
They decided to try and break the caster's concentration with an Attack action, but ended up missing. Being on low health, they decided to risk running away without Disengaging. The caster hit them and they dropped to exactly one hit point.
The dilemma is that if they moved like they are supposed to, they will trigger the thunder damage and drop to 0 hit points; however, they were already moving since the opportunity attack was triggered. So the question is:
Do PCs have momentum? Or are they able to stop on a dime in complete disregard for physics because they just got stabbed?