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There he was ... Meat Loaf harmlessly singing "America the Beautiful" with Mitt Romney at a campaign event yesterday ... when all of a sudden, Mr. Loaf went COMPLETELY OFF THE RAILS.

Meat decided the traditional rendition of the song was a little too boring ... and sane ... so he mixed things up a bit -- with some improv shouting and yelling that he probably considered "creative improvements."

What ensued was nothing short of a musical train wreck.