Dirty water, a too-high pH, overfiring and improper near-boiler piping can all cause the boiler’s water line to surge up and down. If there’s an automatic water feeder on the boiler, it will open and close each time the water line rocks. Before long, you’ll have a flooded boiler.
Don’t blame the automatic water feeder; solve the surging problem by cleaning the boiler, adjusting the firing rate or correcting the improper near-boiler piping.
Well, if you ask about the pressure sensor, This could happen: boiler explosion
Those boiler explosions happened when the over-pressure valve was stuck closed, projecting metal sheets and tubes several miles away !
Even a tiny pinhole in a tankless coil will flood a boiler because the pressure inside the coil is so much greater than the pressure in the boiler. Isolate the coil and watch the gauge glass for an hour or so. If the coil is the culprit, change it.
If you talk about a sensor measuring the level of water in the boiler, then there is two case depending on your water level control system:
The boiler will overflow –
This is not very dramatic if there is no water sensitive devices connected after the boiler. But you will have to clean the steam pipes and all systems flooded with water.
The boiler will run dry –
Which can be bad if the boiler is not thermally protected. If your burner (or whatever device you use to heat the water) only stops when the required pressure is reached, if there is not water it will never stops heating, and you will damage the boiler.