<p>Since <a href="https://stackoverflow.com/questions/253522/do-numerical-primary-keys-of-deleted-records-in-a-database-get-reused-for-futur">DBs do not reuse numbers of deleted records</a> it is possible to run out of numbers, especially if you pick not really a big integer type for this column.<br>
What would happen and how to prevent it if it's bad?</p>

<p>// SQL Server, MySQL //</p>

For MySQL, it is documented that:
The behavior of the auto-increment mechanism is not defined if a user assigns a negative value to the column or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.