The nonsensical phrase must of comes from a mishearing of the contraction must’ve. Except in a few rare instances, must of always bears changing to must have.
Examples
The error is surprisingly common—for example:
… a far cry from the days the journey must of taken in Harry Carter’s time. [Guardian]
… it must of been that Kansas-style wind that set off the alarm. [Brooklyn Paper]
One has to wonder how many drinks this man must of had to try and flirt with the wife of a 7-foot, heavily built monster of a man. [Los Angeles Times]
Kendall must of used his time wisely as his under-3 ERA is a vast improvement over the 12.46 ERA he had in the Gulf Coast League. [Bluefield Daily Telegraph]
Though these examples come from publications that are both online and in print, the error seems particularly common in web writing.
And here’s one of those rare exceptions where must of is actually correct:
Policy on Zimbabwe must of necessity be subservient to this greater goal. [The Age]
