Anytime someone has to type data in a field, bad things can happen.
This occurs in many places. During software development, a transcription error can occur when copying data and formulas into the code. Errors can also happen when copying logic from a whiteboard sketch or other diagram. One must always be careful here, and having good test cases with accurate examples is a must.
For the user, errors can easily occur when they are entering data into fields. Here, you need to add sanity check to the inputs. An SSN should have nine only 9 numerical digits. A phone number should not have letters. Date of birth should not be in the future.
Where possible, you can have the user select from drop down lists to limit options and reduce the chance of error. Alternately text boxes can checked against lists of known good values, if the drop down would be too long (like in the case of state codes).
For more complex fields, auto-complete against a searched table may work. In other instances, a complex regular expression will do. But the more you can avoid a user accidentally typing bad data, the more problems you’ll avoid.
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