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

I believe this is a browser cache issue. Make sure you don't click back and try to change the form, instead, go back then refresh, or click on the edit tab.

Are you using Suhosin to protect your PHP environment?

I was, and I got this error message whenever I tried to submit a complex page (permissions/large menu etc.etc.)

I changed the suhosin.post.max_vars to 1000 (it's in the suhosin.ini file in the conf.d folder for php (normally /etc/php5/conf.d )

This appears to NOT be a Drupal bug, just the security on your server being over enthusiastic.

Another place where Drupal and suhosin don't get along so well is in array depth. I generally set max_vars and max_array_depth to 512.

#240828: "This form is outdated. Reload the page and try again" is incorrect and confuses users (among others)

Thanks elvis2! You solved my problem. I was getting this error trying to save changes to a block, and most instances of this issue referred to Suhosin, so I was in a panic since I'm using shared hosting and can't change those variables.

But your post made me realize that I had loaded the block editing page in my browser yesterday, so caching seemed a likely problem. I reloaded the page and pasted my changes back in and, voila, problem solved.

Subscribing

Do a browser cache-refresh:
[Ctrl][F5] -or- [Shift] and Click the browser refresh button.

Or a direct fresh URL:
Click the address in the URL bar and hit enter to go back to the page fresh. Sometimes the refresh button keeps variables in memory that break things.