Removing the CSRF _CONFIRMATIONTOKEN from page request when passing all parameters to next redirect page. This began causing me issues in Summer '16 release: https://success.salesforce.com/issues_view?id=a1p3A000000jknNQAQ

RedirectApexController.java

public class RedirectApexController {

  public RedirectApexController( ApexPages.StandardController stdController ) {}

  public PageReference redirect() {
  
    // ... do some logic to determine where to redirect to ...
    PageReference page = new PageReference('/apex/MyPage');
    
    // pass any parameters that came in on request on to the final destination
    page.getParameters().putAll( ApexPages.currentPage().getParameters() );
    
    // In Summer '16, a new CSRF parameter is added to URLs, so we need to remove this
    // when redirecting to next page as this won't be valid anymore. One of the risks
    // when wholesale copying all parameters from previous request to next request.
    //
    // If we do not do this then we get error message on visualforce page:
    // "The link you followed wasn’t valid for your session. Please navigate back, refresh the page, then try again."
    // https://success.salesforce.com/issues_view?id=a1p3A000000jknNQAQ
    page.getParameters().remove('_CONFIRMATIONTOKEN');
    
    page.setRedirect(true);
    
    return page;
  }

}

RedirectVisualforcePage.html

<apex:page standardController="Account" extensions="RedirectApexController" action="{!redirect}">
  
  <!--
    Pretend you changed the default View and Edit actions for the Account object
    to call this visualforce page and you have some command buttons that when clicked
    need to do some logic then redirect user to another page.
  -->
  
  <apex:form>
    
    <apex:pageBlock>
      
      <apex:pageBlockButtons>
        
        <apex:commandButton action="{!URLFOR( $Action.Account.View, record.id )}" value="View"/>
        <apex:commandButton action="{!URLFOR( $Action.Account.Edit, record.id )}" value="Edit"/>
        
      </apex:pageBlockButtons>
      
    </apex:pageBlock>
    
  </apex:form>
  
</apex:page>