This is a somewhat unlikely cause for a hung boot issue, and might also be circumvented by changing the boot order in BIOS to ensure USB was at the end of the list, although depending on which of the 4 pins on the USB connector were shorted out. This can occur on ThinkPad T400, T500, R400 or R500 and potentially other models if the USB ports are damaged, causing pins to be shorted as in the photos below.

So if you get stuck on this screen->

Here is an example of a damaged USB port where the tab on the port in the lower portion of the photo has been broken. If a USB cable or device were inserted, it is likely the pins will be bent and might short out, causing the boot hang symptoms described above.

To resolve, you can carefully straighten the pins after powering off the system, removing the AC adapter and battery. Use a small flat blade screw driver, tweezers or micro needle nose pliers to carefully seperate and reform the contacts so they do not touch. The system can then be restarted and should boot properly.

This is a work around, as this port can no longer be used. A complete solution requires replacement of they system board, which may or may not be covered under normal warranty terms, depending upon how the connector was broken.