I was recently surprised by a characteristic of the RPi 3B's USB Output ports. I don't usually monitor the voltage at the USB port, but I was trying a new setup with an Arduino Uno connected via USB while running Windows 10 IoT Core on a headless RPi 3B.
When I powered the RPi I was immediately alarmed because the Arduino's power LED did not turn on and a DVM monitoring the circuit I was testing indicated no voltage. I quickly shut down because I thought I had shorted out the 5V supply. I did observe that the RPi LEDs appeared to operating okay. After a careful check of the circuit I decided to try again but also hooked up an HDMI monitor to the RPi to watch it boot. Everything booted fine and near the end of the process the USB power turned on.
I never knew about a USB power shutdown capability on the RPi. It is probably implemented using the enable on the USB current limiting IC but I haven't been able to find any documentation or schematic to verify that.
I normally don't monitor this voltage at bootup so I don't know whether this is just a characteristic of Windows IoT Core. I'll have to try later with Raspbian. Also not sure if this applies to all RPi models with a USB current limit. I know that the RPi 3B+ uses a dedicated PMIC but I'm not sure where this current limit resides. I'll need to try that too.
I would have been using an RPi 3B+ but Windows 10 IoT Core is not released for it yet.