I was under the impression that the "t" was originally a misspelling

FWIW, the t in often (NB, good historical note), alteration of oft (from Old English oft from PIE *up-) is etymologically sound. It just wasn't sounded often until recently. Spelling influencing pronounciation happens often in English: e.g., perfect was pronounced parfit in Middle English when it was borrowed from French, but then the etymologically-based spelling took over (another French fashion,, and now that c is pronounced.

Ceci n'est pas un seing.