This never happened
Jace, Clary, Isabelle and Simon were heading over to Alec's and Magnus' apartment for the evening. Magnus had invited them for an evening of drinking, games and most likely stupidness.
The four Shadowhunters had nothing to do considering Clary and Simon had both completed their training.
They went to the local store and brought 6 bottles of wine, a case of beer, two bottles of vodka and all the supplies to do suicide shots.
When the girl at the cash register gave them a weird look Jace explained they were going to a party and the girl relaxed.
They loaded their supplies into the back of Izzy's new Porsche and drove to Alec and Magnus' apartment.
When they got there they unloaded their items from the trunk and carried them up the stairs. Jace has a spare key to the apartment because Alec was his parabatai and trusted him enough to have the spare key.
They unlocked the door only to pause in their movements and gape at the sight before them.
Alec was lying on the couch with Magnus sprawled on top. They were both completely naked and obviously in the middle of 'sexy times' as Magnus always calls it.
The couple screeched when they realised they had company and as quickly as they could grabbed their clothes off the floor and redressed themselves.
"What are you guys doing here? We told you to be here at 6 its only just 4!" Alec exclaimed once he finally had his clothes on.
"We got bored and decided to come early. Oh my god I think I'm going to have nightmares for months!" Simon replied as he curled his head into Izzy's shoulder.
"Oh shut it Simona you're just jealous that you don't have a gorgeous naked man on your couch" Magnus sniggered from his place on the couch. Alec turned to look at his boyfriend and slapped him on the arm before returning to his friends.
"This. Never. Happened!" He said and Jace was quick to second that. Needless to say that night started out very awkward but after everyone had drank a bottle of wine each and taken 5 shots no one even remembered their own name.
The night ended with Magnus in Clary's clothes, Jace only in his boxers, Alec dressed up in drag, Izzy and Clary both wearing lingerie and Simon in a vampire Halloween costume.
They would laugh about it in the future but when they all woke up the next morning they regretted every decision they made the night before. They would all have the fun of dying of embarrassment when they realised what they did the night before.
"Uh-Oh" Clary said and ran to the bathroom and emptied the contents of her stomach.
Jace, Izzy and Magnus managed to keep themselves from vomiting but clary, Simon and Alec were not so successful.
When they had all drank some water and taken some tablets to calm their hangovers they all looked at each other and spoke at the same time.
"THIS. NEVER. HAPPENED"