Due mainly to parting to hard, getting to out of it ,getting laid, getting lost, talking to someone interesting, or any other reason . If you go to enough festivals you are going to miss a band you wanted to see, so far I have missed a few over the years including The Who, E L O, James Brown. The stranglers and First aid kit (both airplane malfunctions) Feel free to share your list.
I think I win this topic easily with my 2001 Roskilde-catastrophe. On that single festival I managed to miss (due to drinking and other activities) : Beck, Neil Young, Bob Dylan, most of Tool, PJ Harvey, Nick Cave & the Bad Seeds, Manu Chao, Patti Smith, The Cure, The Hellacopters and lots of bands I later started to like. Still can´t laugh at it.
Every year I end up having many misses, cause I want to see almost everything. In recent years in Roskilde I really wanted to see M.I.A., Afrocubism and the first time Janelle Monae played in 2011. But partied too hard the first 6 days of the festival, went into a zombie-mood for the last two days and totally forgot about them playing. After that I´ve been more careful with the boozing on the music-days. Works better that way.
On a smaller scale, this year I missed The Julie Ruin playing in Helsinki. I´m a huge Kathleen Hannah fan so that bummes me out. And tomorrow I probably will miss Angel Haze, my favourite modern rapper, due to some healt issues, but I´ve already seen her live, so that won´t be such a huge problem.