When do I get this error?

The "Unexpected TODO comment" error is thrown when JSLint encounters an attempt to a comment in which the first word is TODO. The regular expression used by JSLint to determine whether a comment is a "todo" comment or not is /^\W*to\s*do(?:\W|$)/i. Here's a few examples:

// TODO: Finish writing about JSLint errors
// todo
// to do
// ... to do

Why do I get this error?

This error is raised to highlight a lack of convention. Comments that start with the word TODO are commonly used to mark unfinished parts of code. JSLint takes the safe approach and assumes that your code is not production-ready if it contains such comments. If you are happy with "todo" comments being in your code, you can pass the todo option to JSLint to tell it to ignore them:

/*jslint todo: true */

// TODO: Finish writing about JSLint errors
// todo
// to do
// ... to do