Validation error, please try again. If this error persists, please contact the site administrator.

I received a Drupal error today, "Validation error, please try again. If this error persists, please contact the site administrator."

There's probably many reasons for getting this ambiguous error. In a moment of stupidity, I forgot to run update.php after upgrading views module and for a while was dumbfounded by this error.

Running update (/update.php) fixed it.

Edit: also try run /cron.php if update does not fix it for you -or- visit http://drupal.org/node/240828 where there is further discussion, you may find an answer there.