Satellite errors themselves are highly unlikely, in that the signal contains the mechanism for its own integrity to be checked. In this way a failed vehicle can be spotted by its uncongruence with the other signals and dropped.
Most alleged GPS errors that I have come across relate to two simply-explained problems:
1) The Map data supplied with most GPS units is not nearly so accurate as the aeronautical data. The precise positions of roads railways and rivers is not guaranteed. For this reason, if you follow a road around the edge of a CTR using the GPS, rather than the physical object, you are in danger finding yourself infringing.
2) The other common fault is with the aeronautical data itself. Much as it is carefully checked, a number of high-profile errors do creep in and catch the unwarey.
Does this cover your errors, or are there others you are thinking about?
-2Donkeys
[edited for typos]
[This message has been edited by 2Donkeys (edited 06 July 2001).]