I don't see where people have mentioned what set up they use in k3b when the failures occur.
In past threads I have seen where people recommended the slowest burn speed for the best burn.
For some time, I have meant to comment on that recommendation.
Most of my machines have used optical drives that I acquired at thrift stores. They usually ether work when I get them or they are dead on arrival.
The one deviation from this is the three LG drives that I bought new. Two are IDE interface and one is SATA. Otherwise, they appear to be the same mechanism.
The reason I bring this up is that, if I set these LG drives to burn at the slowest speed, I get about a fifty percent failure rate in the burns. The brand and type of media also seems to be a factor here.
If I just let k3b choose what it considers to be the optimum settings, I never get a bad burn.
So, for those having trouble, try it using the full automatic settings and see what happens.
Or, at least tell us what model of drive you are using and what speed etc. the burn was made at. Maybe we can find a common factor when the failures occur.