The date - January 27th, 1984.
The spot - The Shrine Audotorium in Los Angeles
The reason - Pepsi's new commerical featuring Michael Jackson and his brothers
Peep this never-before-seen footage:
Michael suffered second and third degree burns on his head and scalp. He was rushed to the hospital, after which he was prescribed several medications. Sources report that it is this incident that kicked off Michael's addiciton to painkillers and his obsession with plastic surgery.
[paraphrased from Usmagazine.com]