The former couple split after two years of dating in May (15), and a month later, both stars caught a flight out of Toronto, Canada after appearing at the MuchMusic Video Awards.
Simpson took to media sharing app Snapchat to post a photo of him next to the model, along with the caption, "When you get seated next to ur (your) ex on a plane", along with a skull face emoji.
And in a new interview with Australian radio station Nova, Simpson admits he nearly flirted with his former love before he realised who it was.
He explained, "I'm sitting there, I see this blonde hair, I'm like, 'Oh yeah, let's do it!' She was like reading a book or something on her phone, she had her head down so you couldn't tell (who it was)... And then she looked up and I'm like (mouths expletive)!"
Simpson continued, "I look at her and I just start cracking up, like and everyone's looking at me, I start cracking up and she's looking at me like, 'What are you doing?' I'm like, 'This is my seat,' and she's like, 'No way!'"
While Simpson has stayed single since their split, Hadid has moved on to dating another pop star, former Jonas Brothers singer Joe Jonas.