Empirical

Symptom SAT is NP-complete (T027). It is the NP-complete problem, the one everything else reduces to. The textbook conclusion is that you should avoid it. Meanwhile: a hardware verification team routinely discharges instances with two million variables and ten million clauses in under a minute. Package managers solve dependency resolution with SAT. Every bounded model checker (T049) is a SAT solver with a front end. The planning community abandoned custom search for SAT encodings and got faster. ...

Symptom The service has four layers. Nobody designed four layers. There were four teams. Or the version that arrives in a postmortem: the outage happened at the boundary between two services, in the retry logic, because each side assumed the other owned the deduplication. Both teams were right about their own component and neither had ever been in a room with the other. ...

Symptom Two measurements that both look like they break the previous post. The first: your Spark job takes four hours on ten nodes. You move it to a hundred nodes and it still takes four hours — but it is now processing ten times the data. T081 said the ceiling was 20×, and nobody hit a ceiling. ...

Symptom You fix a typo in an error message. Four days later a customer escalates: their alerting pipeline greps for the old string and has gone quiet. Or you make a function faster, and a test suite that had been green for two years starts failing intermittently — not because the function is wrong, but because it used to be slow enough to hide a race. ...