Aniston met Pitt in 1998 and they dated for two years before they married in 2000, becoming Hollywood's golden couple.
Fellow party-guest Katy Perry called Kate out for breaking Jen's golden rule, writing "NO SOCIAL MEDIA" in the comments section of the post.
Jennifer couldn't have looked better in the image, which showed off her gym-toned arms to perfection.
Brad's ex fiancee Gwyneth Paltrow was also at the bash, along with the birthday girl's former lover, John Mayer. And, Pitt, who happened to be in town, happily chose to attend Aniston's milestone birthday, according to the report. "Many of his close friends were at the party too", an insider told the publication.
She discussed inviting him with friends and ended up "very happy" when he attended.
Aniston reportedly didn't give Pitt any special treatment, however.
However, she "didn't really pay any special attention to Brad" as she was also "busy making sure all of her other guests had an incredible time". "They hugged and chatted for a bit, but Jen was busy making sure all of her other guests had an awesome time", they said.
But back to Pitt.
Could Pitt showing up to Aniston's party be a sign of a rekindled romance? The two announced their separation a year ago. "We are two best friends who have made a decision to part ways as a couple, but look forward to continuing our cherished friendship", they said when they confirmed their separation.
The person in the know claimed: "The basis of their night together was that they are in a completely good place and had a very nice conversation, exchanging pleasantries with each other". In October, a source told ET that both were doing well following the breakup.