It happened at the 2009 MTV Video Music Awards where Taylor Swift won the award for Best Female Video.
As she started her acceptance speech, Kanye West suddenly appeared on stage, took the microphone Taylor Swift was holding and started saying that Beyonce should win because her video was "one of the best videos of all time." He then gave back the mic and disappeared.
At this point the show had to cut to commercial so she couldn't continue. When Beyonce got up for her award she invited Taylor Swift onstage to finish her speech.
Source: Know your meme