Common but sadly incorrect mistakes I see on the internet and even in print sometimes.
Really you should have learned the difference about when you learned contractions, but too many careless people/idiots get this wrong all the time. They use “it’s” when the possessive situation demands “its”.
Another hopelessly obvious mistake that borders on retardation. We know about “could have”, “would have”, and “should have”, and their contractions “could’ve”, “would’ve”, and “should’ve”. Those are correct. Apparently some fool saw that the contractions sounded similar to “could of”, “would of”, and “should of”, and decided that they mean the same thing.
As a rule of thumb:
Too often people use “effect” as a verb, and vice-versa sometimes.
The first point is where most go wrong. More specifically, quite often in low-quality writing like advertisements and some newspapers, the last comma is omitted so that the list ends up like “shirt, pants and socks” (in this case pants and socks aren’t being combined). It’s such a common misconception that the incorrect way shows up more than the correct style. This snowball effect (not affect) leads more to believe that the wrong way is right.