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

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 ...