Parallax error may happen in that case as you are not looking perpendicularly.

To avoid this, bring a chair and stand on it so that your eye level comes to the same height as his head.
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If you are measuring the height of a person taller than you then the error will happen will that you can tell him wrong measuring because your eyes will be at lower level than his head. by this you'll measure wrong and you'll tell the person increased height than original.
to avoid this you should get something to stand on so that you can see the person head perpendicularly and tell him the correct height.
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