If you have a catastrophic failure during the GL Post (such as database going down or if the UBE crashes), it is possible to leave the F0902 and F0911 out of balance with eachother. Unfortunately, the GL Post does not use transaction processing to guarantee referential integrity between the two files. Most of the time, this strategy is OK. In the case you observed, however, it is quite wrong not to use transaction processing.
A while back, I re-hooked up some of the GL Post logic at a client of mine to use transaction processing on the GL Post. The result is that the F0902 and F0911 will NEVER be out of balance (regardless of the failure). Also I was able to speed the GL post up by 17 times.
Cheers,
Chris