I know this is not appropriate, but it made me laugh so hard.
I'm trying to write as correct as I can, but the GP just proved that these mistakes happen to everyone, even the people that think that it might make sense.
What about non-native speakers? Their point might be valuable, the participation in a discussion really help- and meaningful, but still - these now/know, there/their, you're/your mistakes are common. And not an indication of intelligence.
Edit:
Just to be sure here and kind of answering the posts below: Guys, I'm a non-native speaker as well (in fact, my accent is terrible. I try to write as decent as possible, but that's a different thing). So - my data point, without any backing but my past experience, is that this is indeed an error that non-native speakers do just as well. I didn't want to imply that non-native speakers are idiots per se according to the criteria of this blog.
Interesting. I kind of replied to the first line in an edit above, but the second one intrigues me.
Probably this just proves that my accent is crap, but for me 'then' and 'than' sound about ~the same~, ditto for 'their'/'there' and 'loose'/'lose'. Again - probably I'm just missing something here. Improving my spoken English is definitely on the list of things I want to do..
I don't think it's a question of sound. (I'm french)
'they're', 'their' and 'there' sound the same (spoken or in my head), but their meaning has nothing in common so I never mistake one for another.
How you acquire a language plays here. I learned english fully conscious of what I was doing rather than my organic absorption of french.
http://en.wikipedia.org/wiki/Muphrys_law