In an inventory of tens of thousands of items, we designed the unique ID of each item to be auto-increment (MySQL) - serial in Informix. The requirements have been extended to be able to also assign the ID from a batches of reserved numbers. This is a valid user requirement allowing the use of pre-printed labels on some items.
You can, of course, enter and ID into an auto-increment field, providing it does not exist, but it alters the next auto increment value. That, in turn, causes "strange" sequencing in the database.