worse comes to worst

When something has gotten as worst as it can get. It can get bad to worse and worse to worst, but it can't get worse than worst.

The usage of "if the worST comes to the worST" has been common, but not logical, because you can't get worse than worst. On the other hand, "if the worst comes to the worst" has also been attributed not to the deterioration of the condition, but the general idea that "if the worst really comes to exist." In such cases, it makes sense in its own way.

"Worse to worse" on the other hand, is usually just a mistake.