Let be more thorough with my reply.
After case trimming a burr is often left on the inside as well as the outside of the case mouth. There are many tools available to remedy this problem. Most common tool used by handloaders is the Cartridge Case Chamfer Tool. One of the limitations of the tool that you should be aware of is the 45* cutting angle. In order for the burr to be removed effectively it must cut into the squared off end of the case mouth thereby removing some extra material that otherwise does not need to be removed. Use a light touch to prevent removing extra material. A sharp edge on the Chamfer Tool is a big plus here. If it is dull it will roll the burr down even more. Why I use the Sinclair carbide tool, it stays forever sharp. A sharp edge on the case trimmer is also a big plus here as it will help prevent haveing a large burr in the first place and leaving a bunch of extra material that you have to remove with the chamfer tool.
This is a good place to insert RULE #1 of Chamfer Tools -- NEVER ROTATE THE TOOL OPPOSITE THE DIRECTION OF CUT. (This rule applies to the Case Trimmer also as well as chamber reamers and other precision cutting instruments. Why is RULE #1 so important? Because it dulls the cutting edge and causes ill results in the finished work.
The cartridge neck wall thickness is fairly thin. So yes, if you remove too much material you will shorten the case and it will leave a gasket cutter edge. Don't worry about it too much if you have gone a bit under "minimum trim length", a couple firings will stretch it back out to spec.