Coachella: Ariana Grande and Justin Bieber perform together

The Coachella Valley Music and Arts Festival, more commonly called Coachella, is one of the biggest music festivals in the world, held in the Colorado Desert of California.

This year the festival celebrated its 20th anniversary with a set-list full of big names and exciting events planned.

But, as always seems to be the case with these things, it was the funny, spontaneous, unplanned moments that really made Coachella 2019 memorable.

Here's a few things that happened that you don't want to have missed.

Billie Eilish fan-girled over Justin Bieber

The 17-year-old might have just claimed her first UK Number 1, but Billie Eilish's reaction to meeting Justin Beiber shows that deep down she's just like the rest of us.

Billie has made no secret of the fact she's a huge Bieber fan, even tearing up during an interview when she revealed he'd followed her on Instagram.

During Ariana Grande's set at Coachella, a fan caught Billie on camera meeting Justin for the first time, and before long they're hugging it out.

Justin even gave her a shout out on his Insta story writing: "So this happened... Long and bright future ahead of you" alongside a photograph of him and Billie.

Justin Bieber made his first public stage appearance in two years

Biebs had recently announced he'd be taking a hiatus from music but that didn't stop him making a surprise performance on Sunday, joining Ariana Grande during her final headline set to sing a duet of his 2015 hit 'Sorry'.

Ariana had said there wouldn't be any guests for her closing show, but we can't be too annoyed at her as it seems Justin's performance was unplanned.

Justin told the crowd: "I haven't been on stage in like two years. I came out here, no idea I was gonna be on stage tonight.

"I wore this fly outfit not knowing I was gonna be on stage. I had to get my groove back, get my swag back."

He even let the Coachella crowd in on a secret, saying: "And, by the way … album coming soon" before leaving the stage.

Someone threw a lemon at Ariana Grande

Justin wasn't the only surprise guest joining Ariana on stage - a lemon went flying through the air towards the star during her Sunday set.

It's hard to tell from watching the video footage whether it actually hit her or not, but Ariana certainly noticed, saying "one of y'all threw a lemon at me".

Because Ariana had apparently banned photos being taken of her set, journalists resorted to capturing the incident in pen drawings.

BLACKPINK made history

Will Smith and Jada Pinkett Smith shared this snap with the band backstage on Instagram

K-Pops stars BLACKPINK performed twice during the two-week festival, becoming the first K-Pop girl group to perform at the festival.

Jennie, Jisoo, Rose and Lisa's Coachella performance came off the back of their first US tour date, drawing massive crowds that spilled out of the tent.

It seems they're also notching up the celebrity fans by the day, with Will Smith becoming the latest person to join the BLINK fan club after declaring himself an 'official fan'.

The actor made a surprise appearance on-stage during his son Jaden Smith's performance, and made the most of his backstage access to grab a selfie with BLACKPINK.

Kanye West's live stream went rogue

This was the view Kanye's fans got when they tuned into his Coachella live stream on YouTube

Kanye West decided to do the decent thing and give his fans, who couldn't make to the festival in person, a taste of his set.

The only issue was the live stream on YouTube looked like it had been filmed through a peep hole, or as some fans on Twitter described it, through a toilet paper roll tube.

Kanye wasn't always the main focus either, with the camera switching from a wide view of the singers to close-up shots following a trail of water bottles during his performance of 'Water'.