What steps will reproduce the problem?
1. visit example.com
2. open devtools
3. enter the following code (line by line, don't copy paste)
let foo = bar;  // throws error
let foo = 1;    // throws error
foo = 1;        // throws error

What is the expected result?
foo should be defined or not defined, but not both.

What happens instead of that?
foo has become Schrödinger's variable, defined and not defined simultaneously.

let foo = bar;
(unknown) Uncaught ReferenceError: bar is not defined    << Expected

let foo = 1;
(unknown) Uncaught SyntaxError: Identifier 'foo' has already been declared

foo = 1;
(unknown) Uncaught ReferenceError: foo is not defined




Not sure if this is devtools or v8, but I don't think there's a sane non-devtools way to trigger this, so starting there.