[w1ntermute]

I actually see native speakers make the "should of" mistake far more often than I see non-native speakers make it, probably because it's a mistake based on pronunciation. Native speakers often incorrectly spell based on pronunciation, while non-native speakers often incorrectly pronounce based on spelling. In fact, the (presumably native-speaking) author of the linked blog post makes such a mistake - he uses the word "manor" where he should have used "manner." Frequent signs of non-native English are improper verb conjugation and article placement, rather than something like "should of."

[StavrosK]

100% correct. Also, non-native speakers tend to make grammar errors, use idioms wrongly, etc. They (we) don't tend to mistake "to/too", "there/their/they're" and "your/you're", because second languages are usually acquired through the written word.

[vidarh]

Well, I do sometimes make the mistakes you give as examples, but only as I get very tired, when I curiously start to mix up all kinds of homophones and near-homophones with no apparent connection (particularly curious to me since, as you suggested, I picked up English primarily through writing; it's not like I have a habit of sounding out words. It took many years from I started reading and writing English until I ever used it in a spoken conversation)

For the same reason I can't ever see me using "should of" - it sounds too wrong.

It is rarer for me to make the mistakes you list than mixing up completely unrelated homophones, though, as they're definitively ones I'm extra aware of.

[vidarh]

In parts of England "should of" is in common use amongst younger speakers to the extent that "should of" to me is associated with South or East London more than non-native speakers (though it's in use many other places in England too).

See http://en.wikipedia.org/wiki/Dialect_levelling_in_Britain#Ex...

My four year old might occasionally use it because his friends does, and he knows it drives his mother absolutely crazy when she notices he uses South London dialect features.

[daledavies]

I always thought the use of "should of" is because people like to shorten "should have" to "should've", especially in conversation. This seems to have lead to people mistakenly thinking "should've" when spoken is actually "should of". Not sure how people can read "should of" and not think it makes no sense though.

* my grammar is most likely not great either, so I can't really criticise to be honest!