At the end of the performance, Justin ripped off a piece of fabric from Janet’s costume, exposing her right breast in what was later deemed a “wardrobe malfunction.”
In the ensuing aftermath, Justin escaped largely unscathed, while Janet’s career took a huge hit. Janet was effectively disinvited from the Grammys, which were held the following weekend, while Justin not only got to attend the show, but won the two awards. It seemed at every turn, Justin’s career took off while Janet was continually punished for the incident.
Many people have also taken issue with the fact that Justin (a straight, white man) never apologized to Janet (an African-American woman) for the incident, and she was held culpable for the entire ordeal.
If you want a more in-depth breakdown of what happened in the aftermath of that Super Bowl, Rich Juzwiak does an excellent breakdown here.