One of the most common causes of bad CD burns.
A buffer underrun occurs when the machine doing the burning cannot keep the pre-burn buffer full enough to keep up with the rate at which information is recorded.
If you should suffer from frequent buffer underruns, there are a couple possible solutions.
1. Make sure there are no other processes running that would suck large chunks of CPU time, and avoid doing anything that requires extensive HDD use.
2. Take your burn speed down a notch.
3. Get a better machine.