The original lineup of Black Sabbath was reuniting for the huge benefit concert after some six years apart, and the band didn't have much time to prepare.
Iommi admitted to SXM Volume that he had no idea who Madonna was when he banned her from their rehearsal room.
"It was a bit of a disaster, really, to be honest," Iommi said. "They had a room booked for us to rehearse. We started rehearsing and then we started talking about the old times. So most of the rehearsal was talking."
When the band actually was playing, Iommi recalls it sounding so rough he didn't want anyone to hear it.
"And the time when we did rehearse, it was a closed session, and it would be embarrassing really because I saw these two girls come in at the back of the room. And I said to one of our crew, 'Look, there's people walking in. You better tell them to get out, this is a private rehearsal.'"
One of those interlopers was indeed the "Material Girl" Madonna, who after the release of her sophomore album, Like A Virgin, a year prior was well on her way to becoming one of the biggest pop stars in human history.
The crew member did as he was asked and told the two ladies to leave. He returned and told Iommi that one of them was Madonna.
The guitarist recalls the whole thing being "a bit embarrassing," but not as much so as the gig.
The founding lineup of Black Sabbath didn't reunite again for 12 years, and he said they made the most of the time they had together at the hotel bar.
The guitarist took the stage the next day with a "dreadful hangover."
Photos: Getty Images