Things like the Flash Crash[1] are what occur. Te scary thing in my mind is that error states are the hardest to test for and tend to be multiplicative in cascading severity. The HFT world is hiring a lot of really smart people. We have no choice but to trust that enough of that brain power is going towards addressing those error states.
As a developer, I'm familiar with the fickleness of software. What happens when we see HFT algorithms error in unison? What have we not thought about?