There are 2 common errors made when importing data:
1. The user forgets to convert their file from XLS to CSV (save as CSV)
2. Two of the column labels in the CSV file have the same name (i.e. "Zip Code" and "Zip Code": one is for the insured and the other is for a property)
Beyond that, most other errors are probably just data formatting issues. Issues like: text in phone number fields, excel formulas in fields, invalid email formats in email fields, accounting formats in currency fields, etc. Those previously mentioned are some of the more common ones.
The bottom line is that the data needs to be "cleaned" before it can be successfully imported. "Garbage in.. garbage out"--- we try to minimize the "garbage in" part.