This week I had a new starter fail after two days. My indy installed one on Tuesday (the previous one had failed to engage on the first attempt a couple of times). On Thursday something internal failed so it kept turning after the engine started (and after I turned off the key). I had to disconnect the battery cable to stop it, and it would then spin (but not engage) if I reconnected it. He replaced it Friday, now it's working perfectly.

Sometimes new parts fail.