Redmine

This might occur when you attempt to create a large number of issues in a single database transaction which is however not comitted but rolled back.

With Redmine, this commonly happens when you attempt to copy a project but some issues (or other elements) are not successfully copied (e.g. because of updated custom field constraints or opther data references which prevent a copied issue to be saved). All the data of the copied project are deleted after a failed copy attempt. The issue IDs however are not resetted. This is an effect of how MySQL handles automatic integer keys.

With that being said, you might be able to reset the next ID to a lower value as long as you have no current issues using a higher ID in your database. There are various guides on how to reset an AUTO_INCREMENT ID in MySQL. One can be found at https://stackoverflow.com/a/8923132/421705