Kirsten Dunst Major Fashion No-No At The Tom Petty Show

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Kirsten Dunst
VIOLATED the "Jeremy Piven Guide to Rock Concerts" last night ... sporting a Tom Petty and the Heartbreakers T-shirt to a Tom Petty and the Heartbreakers concert.

Congratulations Kirsten ... you're officially "That Guy."

Shoulda watched "PCU" before the show ...