This happened to me once on an 86. It was the starter solenoid. The copper disk had welded itself to the terminals. I took it apart and turned over the copper disk. The solenoid is still good, 17 years later.
When it happened to me, it took me a few minutes to disconnect it, and during that time, the starter shaft was spinning. It shortened the life of the starter, and I had to replace it, but there was another problem that nobody connected to the original problem.
After the solenoid and starter problem, I began having a problem with Bendix gears (the throwout bearing that engages the flywheel. It would start making a bad noise and eventually crap out. The dealer replaced one for free, and I eventually had 2 more replaced. The dealer was clueless as to why these things were going bad. Finally, I took it to an indy; he said that it wasn't the Bendix gear, but the shaft it rides on. He pulled the shaft, and sure enough, the pressed bearings on the right side of the case were shot, and the end of the shaft was way out of round.
What was happening was that the out of round shaft was causing the Bendix to slip around against the flywheel, and eventually ruin the bearings in the Bendix. A new shaft solved the problem.
We believe the shaft bearing problem started with the spinning of the starter motor when the solenoid stuck. I had always believed that all the problems started then, but the dealer kept telling me no.
So keep an eye and ear out for any funny sounds coming from the starting process from now on. And don't believe the first dealer you run across.
I Doubt, Therefore I Might Be.