Astudy of 317 Java libraries across 9,000 releases found that 14.78% of API changes break backward compatibility. Less than 3% of those breaking changes actually impact client applications in production.
The gap exists because developers introduce breaking changes for new features and technical debt, not because existing clients need them. Methods take 59% of the hits, types 36%, fields just 5%. Most changes never trigger failures.
Teams monitor constantly. The work is figuring out which 3% will cascade through your stack before they do.