Mr. Rewrite's dictionary of idioms says at worst or at the worst mean under the worst possible conditions or in the least favorable view.
He thought this was an isolated mistake, but Mr. Rewrite found more than a few at worse references on Google News, some by publications that definitely should know better.
TF in TX offers this hint for remembering the difference between capitol and capital, which Mr. Rewrite featured in a recent post: Think of the O in capitol as the building’s dome. The A in capitAl looks a little like a gallows. CapitAl punishment, you see.