I seen

I was just in a store and heard a woman tell someone, "I seen him do that." It's like she's getting seemingly complex grammar partially right: she means to say "I've seen him do that." But if she wants to be grammatically correct without creating such a long sentence (though correct grammar is probably not a concern of hers), then she can also say, "I saw him do that." I don't know why she'd want to say, "I seen him do that" when she could use the same amount of words to say it correctly. It's odd, but common. The woman was happily oblivious about her lack of understanding--she was walking along, talking about some guy she "seen" do something. Oh well.