For a previous job I maintained and supported several external facing applications. The user names that were assigned to the external users started with a sequence that contained a pair of zeros. When the user name was assigned, we sent the user an e-mail that contained the full user name along with a spelled out version of the sequence to *hopefully* eliminate the confusion. Sure enough, the most frequent support call was from users that couldn't log in because they typed the letter "oh" instead of the number "zero". Many of these support calls often took in excess of 10 minutes just trying to explain the difference between the two characters!