What are the errors do we get in real-time?
If you look into the question, its really needs to be looked into broad perspective.But I can say the most real time errors occurs due to Data related issues. Consider while creating the tranformation you've been told that u will get phone num in format 783-130-0001 , u will also design the transformation rules for it. But what happen sometimes u will get the format in (783)-130-0001 or only 130-0001 , etc.. Its just an example. There might be a lot.
Its my point of view.