What version of standard?
12.0.1

What operating system, Node.js, and npm version?
N/A

What did you expect to happen?
This code should be considered an error:

switch (foo) {
  case 1:
    let x = 1
    // some code using x
    break
  case 2:
    x = 2 // ReferenceError on runtime but no lint error
    // some code using x
    break
}

What actually happened?
Code was ported to ES6 & Standard at the same time. Originally it looked like this (simplified):

var x
switch (foo) {
  case 1:
    x = 1
    // some code using x
    break
  case 2:
    x = 2
    // some code using x
    break
}

During refactoring, var x was removed which triggers two no-undef errors. First error is solved with let x = 1 but that makes the second error not to report although x is not declared in case 2.

Adding rule https://eslint.org/docs/rules/no-case-declarations prevents this situation from happening.