Corrected entry: Prior to the explosion from Bullion's case (in Valentin's Istanbul headquarters), Dr. Jones has her hair tied up in a bun, but in all scenes after the explosion it's loose, without any time for her to change it (short of the explosion untying whatever's holding her hair up without injuring her in the slightest).
Shay
15th Apr 2005
Correction: That's exactly what happens. It comes loose as she and Bond dive to the floor to escape the explosion, so the impact knocked loose whatever pin or unsecured arrangement she had.