The WORD "y'all" is a part of our daily vocabulary. We don't have the time to be recorrecting it when we use it in every other sentence.
I don't know how many times this has happened to all y'all, but it happens to us every day.
The word y'all dates all the way back to the year 1824 when it first appeared in the Southern Literary Messenger out of Richmond, Virginia. It quickly spread in popularity and is used to this day in much of the South.