TMI had a failure of the pilot-operated relief valve on the pressurizer along with the associated instrumentation which showed the valve had closed.
http://www.threemileisland.org/science/what_went_wrong/
I agree. The failure of the pilot-operated relief valve to close was effectively a small-break Loss of Coolant Accident.
For what it is worth, I spent about eight hours the Sunday following the accident (which occurred the previous Wednesday) analyzing data from the TMI site.