A developer changes a login lookup to WHERE lower(email) = lower($1) so that sign-in stops being case-sensitive. Every test passes. The btree index on email can’t serve a query that wraps the column in a function, so Postgres reads the whole users table, and on a test database with forty rows that’s the cheapest plan anyway. The change ships. A week later someone notices login latency tracking the signup count. The fix is a one-line expression index that would have taken a minute if anything had objected during review.

Mobile testing is harder than web testing for reasons that are partly technical and partly organizational. The technical reasons: the diversity of physical devices, OS versions, screen sizes, and hardware configurations that a production mobile app must run on is several orders of magnitude larger than the browser matrix that web applications face. The organizational reasons: mobile development cycles historically moved faster than testing infrastructure could keep pace with, producing teams that ship without adequate test coverage and accumulate testing debt that compounds with each release.