After careful thorough research, I came across an interesting form of torture that is a centuries-old interrogation method, Chinese water torture was actually invented far from Asia. it was invented as a means of driving a victim insane. 

Chinese water torture involves securing the captives so tightly that they are immobile. Just above them, there is a water spout. The victims watch as water builds up on the spout and drips over their forehead gradually. Most people find the constant drops to be infuriating since their thoughts always wander to the next drop. Some patients have described the sensation as though the water was slowly drilling a hole in their skull as it fell on their forehead.

This method had me thinking of how some people have shoulder tapping as their pet peeve and I thought what if there was a spell that functioned in the same manner as water dripping torture, but instead always felt like someone was tapping your shoulder repeatedly? 

The shoulder tapping charm could be both a form of torture or a funny move to be used during a duel as a form of distraction. The incantation for it would be, Humeroictus (Hum-Ero-ICTOS) which translates from the Latin "shoulder tap".