There are many sources of error and that is why scientists are so careful to control conditions as much as possible and why they repeat experiments so many times before they come to any conclusions.
For example, you may be timing some type of motion and only take two times and average them. If you had gathered more data you may have found that your initial numbers were incorrect. Or you and another person are both timing the same event. One of the timers may be faulty and so you get two different readings.
Thermometers can be off by varying degrees and so two different thermometers may give two different answers. It may be that one is correct and the other not or that both are incorrect. In either case your reading is incorrect.
When using a balance it may not be sensitive enough for the experiment you are doing and it may need to be recalibrated.
Chemicals used in lab experiments often become contaminated because students put their extra materials back into what they think is the right container but is actually the wrong one.
Experiments are sometimes done in plastic baggies and gases produced may escape through holes in the bag, throwing off a material balance if trying to prove conservation of mass in a chemical reaction.
One error could be a faulty hypothesis. If the hypothesis or educated guess about the solution to a problem is incorrect to begin with, then the experiment will not answer the question or problem and the hypothesis will have to be rejected. Another reason could be that there can be equipment problems. For example, if the equipment is not sensitive enough or accurate enough, then, the data that ensues will not be accurate either. One example could be a faulty digital scale to measure weight. Also, if the experiment is designed to not include a large enough sample, then the data that is collected is not representative of a large enough sample for accuracy. Perhaps the chemicals being used in the experiment are older and not as reactive. There are many reasons why an error can occur in laboratory experiment.