This is a common mistake actually, so common that it's not considered a mistake any more.
The fact is that "Thou" is the second person singular, and "You" is the second person plural.

Here's an example:
"Art thou going to the theatre with me?" - That is only one person, it can't be mistaken for.
"Are you (lot) going to the theatre with me?" - Is modern English, this could be either singular or plural, (hence the "lot" which I added to make it plural for this example.)

Yeah, thou arst a poncy bastard if thou actually speak like this, but at least take note.

No offence to god though..

You know what's funny? I just realised I made the mistake I noded about in my title. Har Har.