Looks like this happened during the Euro 2012 Soccer Championships.

From the YouTube description:

Football fans watching a pre-match dance item before last night’s showpiece final between the First XI and the S-League All-Stars were stunned when one of the dancers left the pitch, following a wardrobe malfunction which exposed her assets.

The performer, who could not be identified, was nearing the end of her dance routine when her silver-coloured tube top slipped.  She immediately pulled her top back up and hurriedly left the field in front of more than 7,000 spectators at the Jalan Besar Stadium.