After the recent RL changes, document.write() replaces page content with its argument, which would totally screw users who copied some ancient code to their user scripts, and would confuse a lot of even technically apt users. An option to reduce the impact could be to override this function with a dummy that outputs an error message instead of completely ruining everything.
Reproduceable on betalabs with http://en.wikipedia.beta.wmflabs.org/wiki/User:MaxSem/monobook.js