Back Voltage

USB 2.0 Specification, Section 7.1.2 & 7.1.5

No USB device shall supply current on VBUS at its upstream facing port at any time, a USB device may only draw current. The pull-up resistor D+ or D- may only become present when VBUS is High. This can be verified by measuring the Voltage over Vbus and GND, D+ and GND, D- and GND when the device is not connected with the Back Voltage fixture. This test must be performed twice, first before the device under test is enumerated and the second time after it has been enumerated. All values should remain below 400mV. A common failure is that self-powered devices put their pull-up resistor active even when Vbus is not detected. This will result in failure where the voltage is ~3V on the D+ for Full Speed and High Speed devices or on D- for a Low Speed device.
Another common mistake is that some device that are self powered or battery powered drive back voltage on Vbus.

Setup:
Back Voltage

According to the www.usb.org devices/hubs that fail this requirement can cause upstream hubs and/or PCs to fail.
Some of the failures that have been reported are:
- The PC fails to cold boot due to back-drive voltage effecting motherboard reset sequences.
- Hub fails to enumerate downstream devices due to reset anomalies.
- Motherboard failures to properly resume from suspend state.
- Introduction of device/hub knocks out one or more upstream devices.