I've had a couple of 'overcharges' but not to 100%. I've got it set to 80% and it's gone to 83 or a little higher several times. Once from a rogue firmware install and once when the servers were down. Can't figure why in either case. Now I reboot the MCU after all firmware installs, to be safe.
As was said, going to 100 once and not staying there for hours and days won't do any harm. If you make a habit of it (weeks and weeks), it's when it will hurt. You can drive it down to 90 or less to get back to where you want it to be, or just let the car's vampire drain eat it up (which may take sometime). May be better to run it down to 95 or 90 and then let vampire do the rest.
As for your question, there is no default setting, and something must have just said 'start charging'.