The brittleness problem

Expert systems had a fatal flaw.

Outside their rules, they didn't say "I don't know." They answered anyway, confidently, incorrectly. Ask MYCIN about a broken arm, and it would produce something that sounded medical but was nonsense. The system had no sense of its own limits. It knew what it knew, and nothing told it where that ended.

This wasn't a bug. It was a consequence of how these systems were built: lists of rules, with no underlying understanding of what those rules were actually about.