404 only on ONE post

Okay, so I get 404 error on only one post… why would that be so?

The permalink structure is /%postname%/

Say, I leave
if I leave the slug as

/how-much-do-dentures-cost

it gives me the 404 error.

If I change to
/how-much-do-dentures-costs
it works.

If I change the permalink structure back to %id (or however it is spelt), it works fine and gives me id=873 or something. Changing back doesn’t work.

Basically, it is just that when the slug is specifically

/how-much-do-dentures-cost
that it doesn’t work!