"Southland" star Ben McKenzie accidentally rammed his head into a concrete pillar ... cracking his forehead open while filming the new show "Gotham" -- and the gash is gruesome.
McKenzie -- who plays a young version of Commissioner James Gordon on the Batman-inspired show -- posted the pics online ... and says the whole thing sadly happened on his birthday.
Ben says he hit up New York Presbyterian hospital shortly after and got stitched up by Dr. Sherman.
Good thing it wasn't Dr. Victor Fries (Batman humor.)
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