16U2 toggle PD7 ~2.5 seconds after powerup.  This causes pulse on MEGA2560's reset pin, which briefly disables Mega's output pins... but for some reason doesn't actually reset program execution.

This only occurs when USB cable is connected during powerup.  Given that a USB cable typically powers the 5V rail, this won't actually ever occur; in my testing that lead to this CAR, my USB cable's 5V wire was cut.