You "could of" tried to use better grammar.

[rae-chan-sensei]

I can sort of under stand the "could of, would of, should of," because when speaking and using the contraction (could've, would've, should've), the "'ve" sounds kinda like "of." Depending on the way you speak, I guess. I agree that it's annoying, but often just an honest mistake.  

[YoTeAmo]

I KNEW that it wasn't "should of", but when I suggested that it was "should have" in 6th grade, my teacher actually yelled at me. She insisted that it was "should of" and that's how I've been saying it ever since. Stupid teacher! I think that some English teachers should have to go back to school before they are allowed to teach again. Actually, I think most teachers (not just English) should have to. You have no idea how many teachers say things wrong and don't think you'll notice.  

[Sheep Kitchen]

I think the reason people use "should of" or "could of" rather than using "have" is because of when we talk, we often contract "should" with "have" and come up with "should've".

Annoying mistake, but honest.

 

[Sovea]

It drives me mad as well. I understand why people do it (the sound when speaking, as a couple of people already mentioned, does make it sound like 'of'), but seriously, do people not read? Do they simply gloss over the word "should've" (could've, would've, et cetera), or do they think it's another word entirely? Gah!

...Which reminds me, it also gets my knickers in a twist when people write "ect". There is no such word! "Etc." is short for et cetera, dammit. See where the e, t, and c come in, and in which order?

*wipes frothing mouth* Sorry, I'm probably preaching to the choir here. sweatdrop  

[YoTeAmo]

Sorry, but et cetera IS a phrase. It means (described in Webster's dictionary) and others;and the like; and the rest; and so forth. I've used that phrase for the longest time so I just HAD to make sure it was a real phrase.