The moment when a female's bra straps form into the face and or head of the famous Autobot from the Transformers franchise, Optimus Prime when she wears a tight shirt.
Guy: "Dude, did you see her Optimus Prime?"
Dude: "Yeah, it was bulging out, everyone saw it."
Guy: "Do you ever think she'll know everyone was laughing at it?"
Dude: "No, girls never really know when they have an Optimus Prime."