Today, my boss ask me help about a very weird bug:
// foo comes from elsewhere, and he wants to use it
console.log(foo.stat); // 0
foo.stat = 42;
console.log(foo.stat); // 0
foo actually comes from a webSQL store and was not writable. With the addition of 'use strict'; at the beginning of the file, an exception was thrown and the confusion disapeared.
Conclusion: always put 'use strict'; at the beginning of your files to raise exception instead of failing silently.