Root cause
also: root causes, root cause analysis, underlying cause
The real, underlying reason a problem happens, as opposed to the symptom you notice.
The underlying condition in a chain of events that, once corrected, stops the failure from recurring. Root cause analysis keeps asking why a symptom occurred until it reaches something fixable at the source, rather than something that merely hides the symptom.
Your ceiling has a water stain. Painting over it treats the symptom. The root cause is the cracked pipe upstairs. Until you fix the pipe, the stain comes back.
Agents are fast at making symptoms disappear. Asking for the root cause stops them from hiding a problem instead of solving it.
The root cause is that the reminder job treats 9am as UTC, a worldwide standard time, not the user's local time. So Pacific users get reminders in the middle of the night. Hiding early reminders wouldn't fix that.
Don't just stop the duplicate charges from showing. Find the root cause of why they happen, explain it, then propose the smallest fix.
Accepting the first explanation that makes the symptom go away. The first cause you find is often just another symptom of something deeper.