> Ah, so A and C where both subscribed to B, but during A's processing of the event it assumed C had already processed it and tried to look up some state. Is that correctly understood?

yes, though you're down the rabbit-hole on this one issue. My point (aside from the fact we actually saw this specific issue and it took a long time to correctly diagnose) is that with thin events followed by a http query call-back, You're asking for occasional "eventual consistency" trouble. Data races will happen occasionally - this is inevitable in the design.

At the tail end of the latency distributions, too fast or too slow, or service A is now having a blip, or you now hit the new version just deployed or whatever, things will go wrong by mis-sequencing in surprising and hard to follow ways (example given that you're fixating on) in complex real systems, and it's a win to avoid that chaos entirely, with fat events.