The Impossible Astronaut - S6-E2
Other mistake: After River shoots at the astronaut going into the lake, she drops her arm. The next second, she turns around and the gun is holstered without her having made any movement to do so, and there was no sound made.
Suggested correction: The holster is low enough for the subtle movement her right arm makes to be using it. As for the sound, the removal of her gun doesn't make a sound either. The only sound heard is directly before it when she put her medical scanner away.