No watermelons were smashed with giant sledgehammers last night...because Gallagher suffered a heart attack before he was supposed to go onstage at a bar in Lewisville, Texas.

Gallagher was transported to a nearby hospital, where doctors said the attack was, quote, "mild to serious." They're keeping him there for a few days at least.

Almost exactly one year ago, Gallagher suffered a minor heart attack and collapsed during a gig in Minnesota. He was smashing stuff with his 'Sledge-O-Matic' at the time.

Check out video of last year's heart attack. Things go downhill around the :25 mark

Gallagher is 65 years old.