Drush suddenly stopped working for one site I have a bunch of sites on my local machine and have been using Drush (v5.4) happily for a long time to administer them.

All of a sudden, with no changes made to one such site, Drush reports these errors:


  Command pm-disable needs a higher bootstrap level to run - you 
  will need invoke drush from a more functional Drupal environment
  to run this command.
  
  The drush command 'dis addthis' could not be executed.
  
  Drush was not able to start (bootstrap) the Drupal database.
  Hint: This error often occurs when Drush is trying to bootstrap a
  site that has not been installed or does not have a configured
  database.


Needless to say the site in question is running fine on the local machine with no errors (and is completely installed). Drush works fine for every other site on the machine.

What can cause Drush to suddenly misbehave like this for one particular site?
I cannot answer why this happens, but I have a site that periodically does this with drush pm-enable.  I think this site has trouble doing the nightly module update check, and something bad gets cached.  A drush cc all always fixes it.