It can detonate instead of burning at a controlled rate. The ideal is having powder burn as long as it can add velocity to the bullet. If the burn peak is too early chamber pressure can go wild overcoming bullet inertia and friction. Use a good handbook, make sure the components are correct and follow directions. It takes a lot of expensive technology to do your own research and it ain’t worth it!
On low powder loads, the powder will lay out in the bottom of the casing or brass and leave an air space from front to back. When the primer is ignited, it will not burn the powder from primer end to bullet end like it should, but flash the powder all at once which results in very high pressures like Swampsniper is talking about.
To fix this, in the early days of of low powder charges, a thin layer of tissue paper or wad paper would be used to capture the powder at the primer end. You could load up some sub sonic rounds this way. PLEASE DO SOME RESEARCH BEFORE TRYING... There are powders out there now that replace having to do this.
Back in my younger days (around 13-14 yrs old)with my brother in law (before he was my brother in law) we use to reload some 06 loads for rabbit hunting that we would shoot out of his dads Garands using this method. Would not cycle, but were extremly low velocity. We could load these up for almost the cost of 22 long cartridge back in the day and were as quiet as a 22 long.