Do Not get me started on Tumblr's stupid tagging system! When I started on Tumblr I tagged my pairing posts with the usual '/' (ie, Derek/Stiles) and had no issue searching for tags that also used the '/'. Then one day it stopped working. Tumblr told me that I couldn't use the '/' in tags, and had supposedly Never Been Able to use it. Except how I had. So I checked their info on tags, and they never mention that there are certain characters that you can't use. That bugs me to this day.