The most common copywriting mistake I see (from a grammar point of view) is the use of an article (e.g., an ‘a’) in front of certain words.
Grammar checkers fail to pick these up because some of them are perfectly fine (they’re just not fine in our industry).
Here’s a couple of examples: “I can write a great copy for you…”; “Here is a text that will help…”.
The second most common mistake is to pluralise those words (copies, texts, contents, etc.).
Again, they’re all acceptable as words, they just don’t mean what the copywriter intends.
These mistakes tell prospects two things:
a) the writer is not competent, and
b) the writer is not prudent.
PS. Another bug-bear of mine is ‘softwares’.
PPS. Am I being too pedantic? What do you think?