The number of Errors is a total number of Warnings and Errors during an inport process. Any field that is over the total character limit (most of them 50 except for note with is 500) will throw a warning which goes in the Number of Errors, but warnings will not prevent the contact from being added. This is generally the source of most errors in the situation you are describing.
If this doesn't answer your question, we would need to have your username and the information you are bringing in so we can look closer into the error. Feel free to email our webservices support team with that information rather than post the username on the forums.