Such is the bane of a professional writer—we can’t read anything without grammatical errors and typos jumping off the page—or off the road sign. 

One day recently, as I was on the highway driving into town, I cringed at a sign meant to be helpful, but could have sent me off the embankment. The sign read, “When visibility is low, drive slow."

Someone obviously thought the rhyming was clever. Perhaps. But what struck me was the poor grammar. "Slow" here modifies a verb and is thus an adverb, and should be written as “slowly.”

In our daily speech, “slow” is often used when “slowly” is correct. Sometimes, the speaker doesn’t know he or she has got it wrong. Other times its use is deliberate. This is only one example of the many grammatical errors we
routinely make when we converse.

Consider “who” vs. “whom.” In conversation, we often use “who” when “whom” would be correct. Some people don’t know they’re making a mistake. And others don’t care. After all, using “whom” when no one else does can come across as pretentious. 

The bigger issue here is this: should we write as we should or how people

The easy answer is that we should be more formal in our writing than in our speech. Although in advertising and marketing writing, we take liberties…And in our emails and Web content, we take liberties…In fiction, more liberties
You see the problem. But as soon as you cast grammatical rules to the wind, someone will hurl them back in your face.  
There once was a time that I believed we should write as we speak. So I used “who” in a printed piece, all the while knowing that “whom” was the better choice. Much to my surprise, out of the vast sea of anonymous readers came a phone call, “You had an error in your piece…” 

This is starting to sound like damned-if you-do, damned-if-you-don’t. Well, not
really. No one can fault you for perfect grammar.   
But, if you do choose to depart from the rules, proceed with caution. It helps if you can defend yourself should you need to.  

The fellow who called me on the incorrect use of “who”? I thanked him and expressed admiration for someone who knew the difference.