When do I get this error?

The "Value of '{a}' may be overwritten in IE8 and earlier" error is thrown when JSHint or ESLint encounters a try...catch statement in which the catch identifier is the same as a variable or function identifer. The error is only raised when the identifer in question is declared in the same scope as the catch. In the following example we declare a variable, a, and then use a as the identifier in the catch block:

var a = 1;
try {
    b();
} catch (a) {}

JSLint will also raise an error in this situation but it uses the more generic "'{a}' is already defined" message.

Why do I get this error?

This error is raised to highlight code that may not work as you expect in older browsers. In modern browsers you code will run without any problems but in Internet Explorer 8 and below you may end up with hard-to-trace bugs.

The ECMAScript 5 specification shows that the identifier of a catch block should be bound to a new declarative environment, created especially for the catch block, rather than that of the enclosing scope (ES5 12.14):

The production Catch: catch ( Identifier ) Block is evaluated as follows:

1. Let C be the parameter that has been passed to this production.
2. Let oldEnv be the running execution context's LexicalEnvironment.
3. Let catchEnv be the result of calling NewDeclarativeEnvironment passing oldEnv as the argument.
4. Call the CreateMutableBinding concrete method of catchEnv passing the Identifier String value as the argument.
5. Call the SetMutableBinding concrete method of catchEnv passing the Identifier, C and false as arguments. Note that the last argument is immaterial in this situation.
6. Set the running execution context's LexicalEnvironment to catchEnv.
7. Let B be the result of evaluating Block.
8. Set the running execution context's LexicalEnvironment to oldEnv.
9. Return B.

Effectively this says that a catch block has its own lexical environment to which its identifier is bound. When the catch block has been executed the running execution context's lexical environment is reset to the state it was in before the catch block. The running execution context is the scope in which the catch block occurs. This works as stated in all modern browsers but in Internet Explorer 8 and below the identifer of the catch block appears to be bound to the running execution context's lexical environment rather than that of the catch block. The result is that an existing variable with the same identifer will be overwritten with the value of the exception. For an excellent in-depth analysis of this bug see The "catch" with try...catch by Ben Alman.

To resolve this issue simply ensure your exception parameter has an identifier unique to its scope:

var a = 1;
try {
    b();
} catch (e) {}

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W002. This means you can tell JSHint to not issue this warning with the /*jshint -W002 */ directive.