Of course, a real pilot would never make this mistake, but sometimes someone in a hurry to get airborne might get tripped up.

Consider the following three screenshots:

The Altitude guage has gone crazy!

The screenshots are all taken at KFSO. But since KFSO is close to sea level, and some parts of the runway are actually with a few feet of sea level, the altimeter is reading correctly in only one of these screenshots. The screenshots were all taken at the same location.

What happened? The answer is simple – you, the pilot forgot to click the altitude knob before take off. This could become a “little tricky” if you only bothered to look at the gauge when in flight…
Remember, X-Plane is not a game, it is a remarkably realistic flight simulator!