http://grammarist.com/usage/hanged-hung/
Not original source of this particular knowledge for me though.
Ah, prescriptivism. The climate change deniers of language. The difference between "hanged" and "hung" is a distinction of a verb categories in Germanic languages. There are weak verbs (regular verbs) and strong verbs (irregular verbs). When Germanic language speakers change the tense of these verbs, weak verbs get a (dental) suffix similar to -ed, and strong verbs undergo a stem vowel change known as
Ablaut, or
apophony. The verbs notoriously switch categories over time, and it's a process which has been happening for more than 2,000 years. Funny thing is everyone still understand each other just fine.
P.S. Reading this thread made me realize that I'm Knightmare, so I'm going to do a serious reevaluation of my life and will hopefully not kill myself.