Would you be correct in so doing?
I would think so, being actually a grammatical error does not effectively rule it out as being a spelling mistake, at lest the way I see it anyway.
Such as the usual expression of day and night - people often use the terms day and night to signify night and not night yet in reality a day is 24 hours long and includes night as well.
Like arithmetic is maths yet maths is not necessarily arithmetic I suppose.
There is the concept of proper use and the concept of normal use which are not always both in true alignment.
So both views might be considered correct dependant upon the criteria within which you use it.
In Johns example ref the "American English" I view our English (The Queens English I was brought up upon) I believe to be the correct one. If Americans or others want to use a corrupted pronunciation or spelling then should specify.
In my particular example with the signwriters then it is my opinion that a signwriter firm should take due diligence and produce the correct version (or ask if this deviation was indeed intended for) therefore the firm was making a professional error.
If you or I might make such a mistake whether common or not and it is not our profession then we might readily be forgiven, someone taking money for that service should have been more robust and the situation be avoided.
We all are aware of people, supposedly professional in some instances, calling a Ring Final Circuit a Ring Main and calling a Fused Connection Unit a Spur, even though theses terms are wrong and actually mean something else.