GMC C-Series 2001 Owner's Manual page 68

CAUTION:
It is dangerous to get out of your vehicle if the
shift lever is not fully in PARK (P) with the
parking brake firmly set. Your vehicle can roll.
Don't leave your vehicle when the engine is
running unless you have to. If you have left the
engine running, the vehicle can move suddenly.
You or others could be injured. To be sure your
vehicle won't move, even when you're on fairly
level ground, always set your parking brake and
move the shift lever to PARK (P).
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CAUTION:
Shifting out of PARK (P) or NEUTRAL (N) while
your engine is "racing" (running at high speed) is
dangerous. Unless your foot is firmly on the
brake pedal, your vehicle could move very
rapidly. You could lose control and hit people or
objects. Don't shift out of PARK (P) or
NEUTRAL (N) while your engine is racing.