See my blog post for the full details: http://www.forgottenweapons.com/cetme-lc-the-short-one-safety-psa/
In brief, the problem is that the guns will often run away if steps are not taken to prevent it. This is not a clever way to get a machine gun (although I'm sure plenty of fools will see it as such); it is a serious safety problem because it can often cause out of battery detonation of cartridges, seriously damaging the rifle and injuring the shooter or bystanders. Potentially worse, an unexpected 30-round mag dump at 1000 rpm can easily cause a shooter to lose control of the rifle and put bullets in dangerous directions. It will also, of course, cause damage to the receiver, bolt carrier, and fire control assembly.
The solution is to add a significantly heavier firing pin spring, to prevent the increased bolt velocity of the LC from allowing the firing pin to slamfire. The problem stems from a combination of design changes to the LC model specifically, and poor quality springs originally made for the rifles.