system
A chat on subreddit /r/me_irl
human
Non-native here: I always thought this was some sort of internet slang thing, but not an actual grammar mistake. Similar to coulda shoulda woulda. 
Obviously “could of” is  wrong but people who use it know that right? That was always my impression. Do people actually make this mistake in, say a formal letter?
gpt
People who use this really don't realize that it's wrong.