I have never had a bad experience with expenses! That's part of what stands out so strongly about this incident. In a culture of blameless postmortems--where an operator can screw up a command causing millions of dollars of damage and the question is "how did this process fail", not "who should we pin it on"--to see my employer suddenly fire a bunch of people for something their process should have detected and corrected would be
shocking. It would make me question their motives. I would probably conclude they're looking for even the smallest excuse to fire literally anyone they can get their claws into.
We've had broadly the same experience but drawn very different conclusions here. This is very interesting and I'll definitely think long and hard about it.
EDIT: I have had (accidentally) improperly reported expenses rejected before. That's normal procedure. It would be quite shocking if my employer instead just fired me... Albeit given U.S. employment law it would not be surprising