In a surprise move that turned just about every head in Hollywood, Brad Pitt stopped by his ex-wife Jennifer Aniston's 50th birthday party in Los Angeles Saturday evening.

The actor was photographed wearing a newsboy cap as he entered the Sunset Tower on Saturday evening to mingle with his former leading lady's nearest and dearest.

"Jen was the first guest to arrive around 8 p.m. wearing a black jumpsuit with her hair down," a source tells E! News. "Soon after Jen arrived, guests started pouring in including Reese Witherspoon, Ellen DeGeneres, Kate Hudson, Gwyneth Paltrow, Cindy Crawford, Robert Downey Jr., Jason Bateman, Jennifer Meyer, Brad Pitt, John Mayer, Katy Perry and Orlando Bloom, who arrived on a motorcycle."

"Jen arrived alone but was excited to get inside," the insider shares. "Brad also arrived alone in a baseball cap. He ducked inside with his longtime security detail by his side."

"Guests mingled out by the pool and back bar drinking cocktails and toasting Jen. They had a DJ, white flowers and lots of candles glowing," the source continues. "Everyone was coming up to Jen and hugging her and screaming Happy Birthday. She had some family there and seemed very happy to be with so many good friends."