Starter tends to be an unexpected failure, though it's rare in the grand scheme of things. It's an item that will just crap out unexpectedly. If you hear grinding shortly after start (YouTube starter grinding noise for a reference, text sucks at describing sound), it may mean your starter is starting to stick and you'd want to use that as a warning sign to replace it.
My advice is, if it's still working, don't worry about it.
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