ECMAScript 5 is in its final draft as I write this; It is due to include a strict mode which will prevent you from assigning to the global object, using eval, and other restrictions. (John Resig's Article is a good introduction.)
This magical sanity-saving mode is triggered by including the string "use strict" at the top of your file (or function.) However, in older environments, "use strict" is a no-op. If you add "use strict" and don't test it in a strict environment, you could be leaving a time-bomb of not-really-strict code that will break when it really hits a strict environment.
Which environments actually respect "use strict"?