There are systematic and statistical errors - besides downright mistakes, which your teacher is calling "human error".
Ignoring mistakes ... systematic and statistical errors come in every time you measure something, or use a value that someone else has measured.
So list everything you have measured and pay attention to how you measured it.
How inaccurate was the measurement (statistical error), and was there anything about the method that could have given a consistently high r low reading (systematic error)?