My wife saw this a week ago in an email message.

Problem:
The modifier of “picnic” is incorrect.

Explanation:
My wife got what appeared to be an often-forwarded email message about being safe with mayonnaise, which has a reputation for spoiling quickly when not refrigerated.

The message referred to the popularity of mayonnaise in meal items that are commonly found on picnics.

The message included the phrase “quaint essential picnic”.

A spelling checker would not have caught the problem in this phrase.

Except perhaps to suggest a comma between the word “quaint” and the word “essential”, a grammar checker would not have caught the real problem, either.

The solution comes from saying “quaint essential” to oneself quickly and appreciating that the email message writer probably was attempting to convert an unrecognized word into two words that he or she already knew.

Solution:
“quintessential picnic”