PHP - Can't Add Rows Past A Certain Id Number (127)

I've got a table which has about 70 rows in. The rows' ids start at 1, have a large gap in the middle, then range up to 127. If I try to add a new row with an autoindexed new id, I get the following MySQL error: duplicate entry '127' for key 1.

If I add a new row with a lower id, e.g. 6, there are no issues - it works fine. If I delete row 127, or give it a lower number, I can then add a new row as normal, but then the error happens again for all subsequent rows.

Everything was working fine with the page that adds new rows until it got to 127. Is this some kind of magic number I don't know about?