Last time I renewed mine, got my photo taken at the local post office and...the U.S. passport office REJECTED it.
So, got it retaken at Walgreens, resubmitted the app, and...the passport office REJECTED it because the envelope was torn in transit and the photo was damaged (or so they said).
Third time, the app was accepted.
It was the fourth time I had renewed the original and I had never had such a thing happen!
Even so, the whole (or)deal took about six weeks due to these delays.
At the end of last summer, I had to apply for a visa to enter New Zealand. Took the photo myself and did everything electronically. No worries, no problems, but did wait about a month for their government to say "Ka pai" and grant me entrance.
BTW, got pulled aside while boarding the plane to Aukland, but that was the random "Gotta Keep the Riff-Raff Out" check.
In other words, take nothing for granted. Start the paperwork now.