The word mistakenly is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 21
Highest Scoring Words With Friends™ Play In The Letters mistakenly
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Seven Letter Word Alert: (33 words)
ailment, aliment, alkynes, amenity, amnesty, anklets, antlike, anytime, elastin, entails, inmates, instyle, intakes, kyanise, kyanite, laments, laytime, manlike, mantels, mantles, meatily, mistake, myelins, nailset, nastily, netmail, saintly, salient, seminal, skyline, snakily, talkies, tinkles