What are the possible causes of error?

Well, that obviously depends on what kind of problem you are solving... Causes of error by definition are any things that may cause your results to be less than exact. In measurement, for example, you may use the wrong units - metrics instead of English Standard units. Or you may have rounded off to 1/8 of an inch but the actual result was in 1/32's. In mathematics and science, a very common cause of error is in rounding - you lose accuracy any time you round numbers, and you can cause even larger errors if you round incorrectly - or closer to the decimal point. In psychology or medicine, the cause of error can be misdiagnosis, interpreting symptoms incorrectly, or having incomplete information on which to base a diagnosis. In economics or business, causes of error can be in not having enough data on current trends, or comparing trends for non-related items. To get a better answer, you may need to relist your question with a specific topic in mind.