Validation error, please try again. If this error persists, please contact the site administrator I've been searching google and I'm about to start surfing through the code, but we all know how useless searching through drupal code is.

People are running into this problem everywhere, including on my website today. So I figured it was worthy of stackexchange.

Basically, I run into this error when trying to send the contact form from my website. Others have run into it with a variety of forms.

I want to know what we're running into that is causing this error.
The form token associated to forms created with the form API is a value that is used to verify the data are submitted from a user who filled a module form, and not forged data.  

The drupal_valid_token() function that validates the token is validating it using the user session; this means that, if the user session used when the data are being submitted is different from the session used when the form was created, the validation of the token fails.
This could happen also when a module calls session_destroy().