After building a lightly used rails application for internal use at the company I work for, I got a notification that there was an error in the application. Someone was trying to add a new record and it was failing (500 ISE). I checked the logs and found the following:

ActiveRecord::RecordNotUnique (Mysql2::Error: Duplicate entry '127' for key 1: INSERT INTO...

It turns out that this problem is triggered when the id column is of type TINYINT which has a maximum signed value of 127. This of course causes the above error to occur when record 128 is entered.

I was able to resolve this by changing the column type to INT using the MySQL client.

ALTER TABLE employees MODIFY id INTEGER NOT NULL AUTO_INCREMENT;