Bug
A flaw that makes software behave incorrectly. Finding and fixing bugs is a permanent part of every developer's job.
What it tells you about a developer
Great debuggers reason systematically when the cause is unknown, and their war stories are specific: the symptom, the hypothesis, the diagnostic step, the cause. Small, precise fixes accepted into critical code are the visible trace of that skill.
Bugs appear in all software at every quality level. A regression is a specific kind: something that used to work and broke because of a later change. Debugging skill, meaning diagnosing an unfamiliar failure quickly, is one of the strongest practical differences between developers.