The Messiah star took to Twitter on Monday to comment on the exes' friendly run-in at the SAG Awards with an apparent message for the Once Upon a Time in Hollywood star.
"This is my 'Do not break our hearts again, Brad' face," Monaghan jokingly captioned a photo of herself giving a stern look of warning to the camera.
Fans got behind Monaghan's message, while one user in particular joked that Monaghan "could've stopped" that first heartbreak. Monaghan starred as Gwen in Mr. and Mrs. Smith. Pitt has said he fell for Angelina Jolie while filming the 2005 movie. He and Aniston split that year, after five years of marriage.
"You were on that set, Michelle! You could've stopped it. You were asleep at the goddamn wheel! ?," author Jack Walsh wrote.
After his breakup from Aniston, Pitt went on to marry Jolie, though they've since split. Aniston, meanwhile, married Justin Theroux in 2015, but they've also since gone their separate ways.
A source told ET last August that Aniston was starting to date again with the encouragement of her friends, including Courteney Cox and Lisa Kudrow. The source also said Aniston and Pitt -- who joked about adding his SAG Awards win to his Tinder profile in his acceptance speech at the SAG Awards Sunday -- were still in touch.
"She continues to stay in touch with Brad and they occasionally talk," the source said. "They both have come so far."
At the Golden Globes earlier this month, Pitt told ET he wouldn't mind running into his ex. "I'll run into Jen, she's a good friend. Yeah," he said.
"The second most important reunion of her year? I understand. That was a play on Friends. They were saying that," he added in reference to Aniston, Witherspoon and Christina Applegate who were all at the Golden Globes and played sisters on the iconic sitcom.
On the red carpet at the SAG Awards, Aniston told ET she was maintaining a sense of humor about the commentary on their relationship.
"It's hysterical," she said. "But what else are they going to talk about?"