Just to list a few that are generic:
Equipment - Either wrong equipment used or the equipment does not measure to the proper level of precision. If you used a beaker to measure 30 mL instead of a graduated cylinder you could only measure to .0 mL where as with a graduated cylinder you could measure to the .00mL
Contamination of chemicals if it is chemistry.
did not conduct the proper number of trials normally 3 is acceptable
Rounding of data to early
Humidity exposure of items such as salt that obsorb water vapor from the atmosphere.
It would help to know the experiment but I hope these help.
well, that depends on the experiment. for example wind could be a source of error if your expirement is outside becasue you have no control over it. you could of added too little or too much ingridients in whatever you are doing.