This is still a topic of research.Going below the minimum or above the maximum generates dangerous overpressure.
Too much? Usually boom but not always.
Too little? Sometimes one gets what is called "detonation". Smokeless propellants don't explode, they deflagrate. Difference between the two is the speed of reaction. The rule I heard of was the cut off was the speed of sound in the medium versus the burn rate along a line.
I've never seen detonation. I don't stray too far from manufacturer's suggested loads either.
Some people see things like failure to cycle an action, maybe a round stuck in the barrel. Most of us just see disappointing range results.