In general, I’m not the sort of person who corrects people unbidden, but every time I see a message with must of in it my pedant urges twitch, and I have to stop myself typing a Google-style “Did you mean must’ve?”
Must of is an incorrect way of saying must’ve or must have. The contraction is pronounced like must of, presumably causing the confusion.
It is extremely rare that must of will make sense together in a sentence e.g. ‘They must of course consider what is proper’. Commas would sort that right out, though.
They must’ve been scandalized when she married the chauffeur.
You must’ve mistaken a footman for a valet. How embarrassing.
I think you must’ve watched too much Downton.
Have you seen this happening? Comments and shares much appreciated!