One problem related to steering failed has been reported for the 2005 BMW Z4. The most recently reported issues are listed below.
The car had been sitting outside in approx. 85 degree weather for about 2-3 hours. My wife and mother in law were driving done the highway. She initially noted that the steering felt "sticky" or "different". A few moments later while traveling at approx. 55mph the steering failed to respond or more accurately felt stuck. She was unable to turn the steering wheel and the car started to move to the right side of the road toward the guard rail. After slowing the vehicle and continuously trying to turn the wheel it finally responded. She pulled off the road and sat for a few moments to regain composure and check vehicle. There was nothing outwardly apparent, no flats, nothing under car, nothing wedged in engine compartment or underneath car. She exited the highway and drove slowly back to the house. The car did exhibit signs of "sticking" again, but did not lock up. A quick search on the internet has revealed this is a very common complaint on the Z4. This is a very serious safety concern and now my wife is afraid to drive the car.