I was at Dashboard >: Plugins > Add New and searching for a plugin that happens to have an apostrophe in its name (Peter's ...), and I received this error:
403 Forbidden Error Page
If you arrived here due to a search or clicking on a link click your Browser's back button to return to the previous page. Thank you.
Undefined variable: date in .../wp-content/plugins/bulletproof-security/403.php on line 63
Removing the apostrophe from the search resolved my problem, but I thought you still might like to know.