Combat points will never be lost as you can always assign any remaining damage to your opponent.
With the exception that if your total combat does not exceed the total defense of your opponent's Guards. In that case, combat does get lost.
For example, if you have 4 combat and your opponent has a Guard with 5 defense, then those 4 combat are lost at the end of your turn. You cannot hit your opponent with it, since your opponent is protected by that Guard.