In this case the misspelling is actually the correct one. HN has shortened the link in GPs post but if you follow it you'll see it spelled "Muphry's law", and it is a separate one from the general one that everyone knows about.
That said it wouldn't surprise me at all if I've misspelled something here : )
The correct usage would be to talk about an error "in" a sentence, not "on" a sentence.
Muphry's law get you every time!
https://en.wikipedia.org/wiki/Muphry%27s_law