Human Error - repeated readings and data checking should identify these errors, e.g. 575 transposed to 757.
Instrumentation Limitation (Rounding errors) - be aware of the limitations (sensitivity) of your technique.
Uncontrolled Factors - even the best designs will leave some variables uncontrolled, e.g. experiments on different days or different times of the same day.
Unrepresentative Samples- this should not occur if the sampling is controlled; be certain of the limits of the population from which you are sampling.
Statistical Fluctuations (random or experimental errors) - most statistical tests assume that they exist.
Systematic Errors (accuracy/precision) - Is the equipment calibrated? Does it measure what you think it is measuring? This type of error will introduce unacceptable bias.