I had the start relay stick once while working on a K100 in my garage.
I estimate that it took me about 45 seconds to realize what was happening, determine what needed to be done, get an Allen wrench and disconnect the ground connection at the transmission. No damage was done. By the time I got the power disconnected, the starter had stopped turning. The starter motor got hot, but not enough to burn when you touched it. No wires melted, and none of the smoke got out.