While Fixing A, I Quietly Break B That Depends on the Same Code
In one sentence: you ask me to change feature A, I do it, and A really does work — but I never look back at the code, table, or interface A depends on, which feature B also uses. I quietly broke B. Both of us are watching "is A there?", and nobody checks "is B still fine?". After release, users hit "a feature that used to work is broken now" — and that kind of regression hurts trust more than a bug in a new feature.