Attachment ID column setting

[Oldiesmann]

I've seen this type of thing happen twice with two different versions of MySQL. At some point, the Auto-increment thing stops auto-incrementing past a certain value until you change the column type to BIGINT or increase the length.

Most recently, this happened on an SMF board with 1483 attachments/avatars (the users love that feature apparently). It wouldn't auto-increment past 1653 despite the length being set to 11. I increased the length to 13 and now it's working again despite the fact that 1653 is nowhere near 4¹¹ (4194304) and is much much less than 4294967295 which is the maximum unsigned value for that column type.

Does anyone know why this is happening? The version of MySQL that this most recently occurred on is 3.23.57, but a similar thing happened to me with my shoutbox (smaller setting) and my host is running 4.0.20.