tedster - 11:08 pm on Oct 21, 2010 (gmt 0)
It happens all the time, during major back end updates and data movement, for example.
The servers that publish the public cached links are not the same as the servers where the actual ranking analysis is done. Sometimes a public cached page is not available or reachable. And sometimes there still is one, but the date goes backwards from what is clearly indexed, even by several weeks.