This PHP script will check all MySQL tables and warn you if any integer fields with an AUTO_INCREMENT setting are approaching the limit of that field's datatype.
When the limit for a field size is reached future INSERT operations will fail (generating an ERROR 1467: Failed to read auto-increment value from storage engine) resulting in an annoyed user. Set up a cronjob to run this script regularly and email the results to receive a warning in time to increase the limit or take other action before any damage is done.
Yes, a good coder should be able to choose realistic datatypes for the data they will be storing allowing for room to grow, but I don't like to leave things to chance!