I wonder if using cheap inner tubes near the upper limit of their size range predisposes them to failure. It seems logical that the more the tube has to inflate, the thinner the rubber will be. This may be desirable for keeping rotating mass to a minimum, but there's less rubber to resist punctures or seam separation.
I recently bought four "Serfas" tubes from PerformanceBike. They were the right size ("700x28/38c") for the tires (700x35), but the first two I installed went flat shortly after inflation. One had a hole on the sidewall area where there was no sharp edge inside the tire. I haven't seen where the other one went flat because it's at my son's house. To PerformanceBike's credit, they sent me two new ("Specialized") tubes.