Cameron Diaz & Jessica Biel

When run-ins aren't planned, they are far less likely to go smoothly and civilly. Case in point: shortly after Cameron Diaz split from longtime boyfriend Justin Timberlake, she ran into him at a Golden Globes after-party, flirting with Jessica Biel (who ended up becoming his current longtime girlfriend). Instead of acting like an adult, Cameron freaked out at both Justin and Jessica in front of Hollywood's elite. Oops.