Some time ago, a regression was introduced into the 3.13 line kernel used by Ubuntu 14.04 LTS that broke UDP sockets when they were used in a certain way (e.g. like FreeRADIUS does). This bug was fixed in 3.13.0-67 and I hoped to never see it again.
Two days ago, I realized that one of our RADIUS servers was not working correctly any longer. I could not tell how long this problem had existed (the second RADIUS server still worked and in monitoring the primary one also worked, so the problem went undetected for a very long time).
After looking for the cause of the problem for quite some time, I remembered the problem described earlier and tried an old kernel version. Bingo! This fixed the problem. After looking at the changelog of the current 3.13 line kernel from trusty-proposed (that also fixes the problem) I found a reference to another bug report that describes the problem (don't be fooled by the bug's description, it also applies to IPv4).
As it turns out, the first regression had been caused by backporting an optimization regarding UDP checksum calculation from a newer Linux kernel. However, this change exposed a problem that had been fixed in the newer kernel but not in Ubuntu's branch of kernel 3.13. This regression was fixed by simply removing the patch again. This was okay because it was just an optimization.
Some time later, someone (who obviously was not aware of this regression) again thought that backporting the optimzation was a good idea, so it got reintroduced in 3.13.0-69. Now, it looks like they fixed the bug in 3.13.0-78 by actually fixing the underlying problem and not by removing the patch again. Therefore, I hope that we will not see the regression a third time. However, I am a bit annoyed that they did not do better testing when backporting the patch after there had already been a regression around it once. Maybe the Ubuntu team's decision to not use a kernel with long-term support and do maintenance themselves was not so wise after all.