[The Squatter of Amon Rûdh]

That was well found, but it's still following the normal usage of 'mischief' as established in the seventeenth century and found in any dictionary. Something tells me that Tolkien was up to something here, and a glance at A Middle English Vocabulary confirms this. In his gloss for myschefe and its variant spellings, Tolkien gives the Old French form as mesch(i)ef, clearly identifying it with chef. It would be just like him if his research into a word's origins affected his use of it in modern English.