User Error
Noun · Slang & Abbreviations
Definitions
A mistake made by the person using a system rather than by the software itself, though good design practice recognizes that many so-called user errors are really product design failures. The phrase can be accurate or unfairly dismissive.
In plain English: A mistake made by the person using the system.
Example: "Yes, it was user error, but the UI also made the wrong action much too easy."