It's a strange festival tradition: urinating into bottles or cups and hurling them in the direction of the stage.

Such a shower of disrespect has a history at Reading Festival.

Bring Me the Horizon were bottled in 2008 when they appeared as a last-minute replacement for Slipknot.

My Chemical Romance suffered a similar fate at Reading in 2006, the same year that Panic! At the Disco lost their singer momentarily.

Brendon Urie crumpled on stage after a plastic bottle hit him in the head, knocking him out just 30 seconds into the band's first performance at Reading.

He says he's proud he managed to come round to finish the set and has happily returned to play the festival several times since.

"The bottle was a good omen, it boded well for the future," Brendon tells Newsbeat.

"It was baptism by bottle. It was our first introduction to a festival crowd in the UK and I immediately had an appreciation for just how mental everyone goes. That's something you don't get so much in the States, that's a different kind of mental."

It's Run the Jewels' first time at the Reading & Leeds festival and as Newsbeat unwittingly shares the folklore of bottling before they go on stage, the duo shows a healthy apprehension.

"Respect the tradition," they say.

"Like shake my hand after you pee maybe," says Killer Mike, "but don't throw pee at me please".

El-P is more practical: "The best way to disarm that whole thing is to just run out there and pee all over yourself. Just take the wind out of the whole thing."

Years & Years have never been hit by a bottle but tell Newsbeat: "Iconically it happens at Reading."

"I can tell you it's not unique to Reading though," adds singer Olly Alexander, "because it happened to us at T in the Park. No-one chucked it at us but I could see so many bottles flying in the crowd."

Over on the alternative stage, Russell Kane's just finished a stand-up show and is having his post-gig glass of red wine.

Russell tells Newsbeat: "It's the ultimate heckle isn't it? To have someone's feculence thrown at the stage."

He says it's less of an issue for comedy.

"It's the only arena (at the Reading & Leeds festival) where people sit cross-legged, so anyone standing up causing trouble is quickly ostracised.."

It's been happening elsewhere on the site though.

Festival-goers Steph and Sophie are from Reading and are both 16. They tell Newsbeat: "We've seen it in the dance tent, when DJ Fresh was on and Charli XCX. It didn't make it to the stage, it was just in the crowd."

Maddie, 16, from Surrey also witnessed someone trying to throw bottles at the stage.

"At Royal Blood, this guy filled about three bottles full," she tells Newsbeat.

So what about the throwers themselves? We found a few festival-goers who would admit to doing it, but they wouldn't give their names.

"I do it all the time," one woman said. "You just gotta do what you gotta do. It's just a bit of pee, no harm done."

You can watch full sets and videos from more than 40 acts at bbc.co.uk/readingandleeds.

Follow @BBCNewsbeat on Twitter, BBCNewsbeat on Instagram, Radio1Newsbeat on YouTube and you can now follow BBC_Newsbeat on Snapchat