What is wrong with this sentence from a mystery shopper report?
The service could of been faster.
There are a couple of problems. Not only does it express an opinion rather than an objective fact, it does so badly.
There is never a case where “could of” is a grammatically correct phrase.
The same is true of would of, should of and must of. And, even worse, coulda, woulda, shoulda, and musta. Never even think about using them in your reports.
The correct phrases are could have, would have, should have and must have.
So why do so many people use “of” instead of “have”?
Probably because when people use a phrase such as “could have” in a sentence, they tend to shorten it to “could’ve.” People who are accustomed to hearing the spoken phrase hear it as “could of” and therefore write it that way.
The grammatically correct version of the above sentence would be:
The service could have been faster.
Of course, it is still opinion and thus probably does not belong in a mystery shopper report. However, there may be times when the use of “could have” is appropriate in your reports. Just make sure that when you use it, you spell it correctly.