combo box fills in arbitrary value

A combo box on a form selects from a list of codes (001, 015, 400, 411, 500
etc). These are formatted as text with an input mask of 000. The Row Source
is a simple Select statement that draws from a table.

When you start typing the first character in this box, Access auto fills
after the cursor with the value '411'. So if I type 0, the value is 0411,
which is not even a valid code. Even if you type 4 as the first character it
auto fills with 4411. Doesn't seem to make any sense. I've tried changing
every combo box property (I think) including those having to do with
defaults. Nothing seems to change this behavior.

I use many combo boxes, and only a few have behaved like this; I can't
figure out why.