Earlier quoted context omitted.
In my experience, going past the max fill line is mostly a problem when using the “rapid pressure release” on a recipe with lots of liquid. Pressing the release button can emit a a superheated geyser of soup with enough force to hit the ceiling.
When the timer goes off and the cooker switches off, the contents are still under pressure. That pressure shifts the liquid's boiling point, which is why pressure cookers cook hotter. If you vent the pressure vessel, the liquid inside will rapidly boil as it dumps that excess thermal energy to steam and your kitchen. It'll carry bits of liquid and cooked food with it. Another thing you know but may not have connected…
You don’t want to be nearby when this happens.