RJ-45 and USB ports are exactly the same width. If you plug in a USB port blindly, you can feel it make contact with the sides of an RJ-45 port.

I found this out trying to fix someone's printer. I even un- and re-plugged the cable and it still wasn't detected. It wasn't until I actually looked at the back panel that I noticed the printer's USB cable was plugged into an RJ-45 network jack.