Which astronaut slipped and injured his head in a fall in a shower on 26th Febuary 1964?

john Glenn.

On February 26, 1964, a bathroom rug slipped under him in his Columbus, Ohio, apartment, and Glenn fell, striking his head on the tub. He received a concussion which affected his inner ear balance. This injury produced swelling and bleeding which upsets the delicate and sensitive equilibrium mechanism. Glenn suffered from persistent and disabling symptoms of dizziness, nausea and a ringing noise which markedly restricted his physical movements.

http://space.about.com/cs/formerastronauts/a/glennbio_2.htm