Keep your ear the ground at Glastonbury and you could be in for a major treat. Secret sets - when major bands pop up unbilled on smaller stages - can be the highlight of the festival.
Last year, The Libertines touched down in Pilton, Somerset, in a bright red helicopter to perform a surprise slot, while The Charlatans opened the festival with a cloak-and-dagger gig.
It is all about knowing what to look out for: an unannounced artist kicks off proceedings on the Friday morning on The Other Stage, while William's Green usually has two surprise acts on Thursday night, warming up for their bigger slots over the weekend.
Some interesting slots in the official schedule that may prove fruitful are late-night Sunday at Arcadia and a daily gap on the BBC Introducing stage, which is known for bringing back famous alumni.
To whet your appetite, here are some of the best secret sets in recent years.
Radiohead - 2011
There have already been whispers the Oxford band might turn up to this year's festival, with a convenient gap in their touring schedule for new album A Moon Shaped Pool.
Radiohead are no strangers to turning up unannounced, although it was the year's worst kept secret when they hit the Park Stage in 2011 to introduce album King Of Limbs, getting the crowd on their feet with Street Spirit.
Pulp - 2011
Just the day before, Pulp had co-opted the Park stage for a sun-soaked riot through classics including Disco 2000 and debuting Like A Friend live.
Jarvis Cocker - whose on-stage chat ranged from a happy birthday to George Michael to the crowd-pleasing line, "Glastonbury is not about me, it's not about you, it's about us" - ended with Common People to roars from the audience.
Franz Ferdinand - 2008
The set was even a surprise to the band when Franz Ferdinand spontaneously took to the Park Stage because they happened to be at the festival on a day off from their tour.
Frontman Alex Kapranos shouted "Surprise" at the crowd before launching into a rousing selection of hits like Walk Away and Take Me Out.
Mumford and Sons - 2013
Following on from their Other Stage performance, Marcus Mumford and his banjo-wielding band did an impromptu five-song set in Strummerville, the area named after The Clash's Joe Strummer, with campfires flickering in the background.
The Last Shadow Puppets - 2008
The side project of Miles Kane and Arctic Monkey's Alex Turner brought on board Jack White on guitar (who they happened to meet backstage) and Alex's Monkey's bandmate Matt Helders on drums for a once-in-a-lifetime performance of stripped-back hits.
Skrillex - 2014
The dubstep DJ made an unplanned detour to Arcadia after his main stage performance, pumping tunes from the 360-degree DJ booth inside the colossal fire-spitting robotic spider.