We all have a go-to karaoke song. Whether you like making the evening fun by badly shouting the words to something upbeat and cheerful, or you're one to show off your impressive vocal range by attempting a Whitney Houston classic, karaoke is always a fun night - especially if you're with your mates. But one woman got more than she bargained for when she took part in a bar's karaoke night and chose to sing a pop hit , because a mystery guest appeared on stage to sing it with her. A video of the