Hanged - hung
In the days when capital punishment was still part of the law in the UK, the past forms, both past tense and past participle, used by careful speakers of English for a person executed by order of a court was hanged. Burchfield's Fowler says that many writers have used hung for executions, and "This use is not erroneous, but just less customary in standard [sic] English". Historians and similar writers might try to preserve the older form; others are unlikely to need to.
The other obsolescent use of hanged is as an older imprecation, or (mild) oath. "I'll be hanged if I will do that" was a forceful way of saying 'no' in the past. Students of Literature may need to quote this usage from time to time; otherwise it is dead.