On my 71 Bee - This scenario drove me crazy for more then a few summers
Remember , It only happened on occasion
The factory or reproduction positive battery cable that runs down to the starter
I would start the car , engine running , and the starter would still stay engaged on occasion
Shut the key off , engine off , and starter is free wheeling
Remove the hood pins , pull the hood release , pop off the negative cable
Went through numerous starter relays , ignition switch , checking grounds , bulkhead connector , blah blah
Test lights - Multi Meter - Wife crazy in the car
One day I just said , car is not leaving the garage - Started removing all the vinyl wrap on wiring harnesses from the starter relay thu bulkhead - Removed the starter and positive cable assembly
Took the wrap off the reproduction positive wire cable , and was like , really that simple , but hidden well
Here internally , the smaller positive wire that energizes the starter solenoid off the starter relay was grounding out on the factory frame bracket (The black rubber coating for the two prong bracket was worn off) for the positive cable holder down by the frame , melted internally the insulation on the large feed positive wire for the starter along with the insulation on the small feed wire
And when I hit the key , and on occasion , combustion LOL
And mostly all covered up by the black tape and black wrap when they made the reproduction cable for a 71 B Body
Had me scratching for many car shows and in the garage every time I went to start the car
Took that factory bracket off , blasted and dipped in that rubber tool coating on the prongs
New reproduction positive battery cable
Amazing ever since