Good thing Harry Styles didn't go commando.
It was midway through "What Makes You Beautiful" that Liam, 19, surprised Harry, 19, from behind, giving the excited crowd a full glimpse of the boy-bander's tiny boxer-briefs. Ever the professional, Harry carried on with the song, grinning as he pulled his trousers back up.
Butt, this harmless pantsing prank wasn't all that went down during Saturday's show. Louis Tomlinson took a crack at The Wanted, joking that Jake Bugg, an English singer, had "the best gig ever" supporting the rival boy band. (Harry, adorably, tried to defuse the situation, saying, "Oh, come on! We're all friends.")
This got Tom Parker's shorts in a bunch. The Wanted singer tweeted (and then deleted), "@Louis_Tomlinson you even talk about us at your own gigs. Are y ou that upset you didn't get in this band?"
Louis tweeted back at Tom, writing, "Pal, we both know I wouldn't waste my time auditioning for your band. You humour me with your bad boy persona."
He then went on to tweet a link to Tom's failed X Factor audition and mock various tweets of The Wanted star. Tom responded wrote (and has since deleted), "Yes @Louis_Tomlinson, I don't need to call producers to beg my way on to a track over my 'friends' #crybaby #loyalty #louisgottalent."
Louis, 21, also taunted Tom, 24, writing, "Interesting you have to entertain your many followers with lies. I actually think you could be the biggest twat on twitter."
Liam jumped in, tweeting, "hey Tom lets talk about your singing your amazing tone pierces my ears with every note :/" He also added, "p.s. tweet out when you have someone to speak to our drummer has more followers than you."
"I also heard he wets the bed," Louis responded.
Louis sarcastically urged Tom to "make your next single our movie soundtrack." Tom responded (and yes, then deleted) that he'd "love to make the soundtrack to Mean Girls 3."
"@Tom TheWanted not a bad idea actually," Louis sniped back. "maybe @maxthewanted could ask Lindsay [Lohan]?" (Watch yourself, Louis.)
The Wanted's Jay McGuiness chimed in with a lengthy, punctuated and grammatically correct tweet. "Dear @Louis_Tomlinson, please stop mentioning us in your gigs," the 22-year-old Brit wrote, "we certainly no longer mention you and it's time to let dead dogs lie. Your passive-aggressive style of speaking makes me cringe, and I wish you'd either have the bollocks of some of your co-workers to speak truthfully, or the class of the majority of them to be silent.
"I'm not sure what's happened since we saw you at the XFactor, but you've done a sterling job of becoming one of the most overrated, arrogant and not to mention insincere people around," Jay, 22, added. "Your shocking lack of talent will only be forgiven by lots of humility and no-strings-attached friendship among your band. Louis, don't measure your worth in followers or money, because they're fickle, and when they go you might just feel worthless."
Louis wrote back, "@JayTheWanted this would be a lovely insight for your next book. You must have been a splendid student. Never forget you boys started this."