Why didn't anybody tell me about Redis hash slots?
A backend engineer at a gig-economy delivery service hit a wall caching route estimates in Redis. Their keys, built from H3 hexagon IDs, never landed in the same hash slot, so every MGET exploded into dozens of round trips. Redis hash tags, brute-forced integer anchors, slot-aware batching, EVAL for expiry, and dropping JSON for CSV fixed it. The lesson: build the benchmark harness first.
I think in the post-Claude era of programming, where such a benchmarking harness is a matter of providing a spec and waiting 10 minutes, this is no longer acceptable.