Thinking about your everyday, informal manner of speaking, I would like you to read the following sentence, and then pluralize it:
You don't know what you're talking about.
Just leave the result in a comment, along with any demographic information you wouldn't mind leaving, up to and including age, gender, location, etc. And try not to read other comments before submitting yours. This is the best way I could think of to ask this question, and there is a reason behind this that I'm contemplating.
ETA: Thanks for everyone's responses. It struck me last night as I was saying this very phrase ("y'all don't know what you're talking about") that it would have sounded odd, to me at least, to double up the y'alls, something that is unusual for a standard pronoun. It doesn't sound as bad as "Frank doesn't know what *Frank's talking about," but the fact that for me, it sounds odd to double it, led me to suspect "y'all" of being something other than a mere pronoun. A majority of the responses say it the same way I did and would, but enough people replaced the second "you" with "y'all" as well that I'm not entirely convinced.