We were departing MIA headed for DFW on an AA DC-10 when a young man was escorted on board. He soon began yelling "no! no! the bomb!" and considerably more as his apparently psychiatric escort turned green. People in uniform boarded, and he began shouting "no! no! I want to go!" And, off he went, with the people in uniform.

I felt sorry for the chap - his mental health providers had not properly calculated the extent of his anxiety or Tourette syndrome perhaps. He was definitely not fit to fly, and he caused some anxiety in passengers who didn't have an idea what was going on with the shouting about bombs and all.

More commonly, I have seen passengers involuntarily disembarked when an aircraft was overloaded (high density altitude, insufficient runway length, too much weight in passengers, cargo and fuel).