title: FOSUserBundle sending bad confirmation links
body: <p>1 or 2% of users registering seem to get no confirmation email or they get a bad token appended to the confirmation link in the email. I can't determine why or where this wrong token comes from. It doesn't look like a truncated version of the correct one. It's totally different. Clicking it yields a 404 response.</p>
  
  <p>I am not overriding the RegistrationController.</p>
  
  <p>Could it be the mailer settings?</p>
  
  <pre><code>swiftmailer:
      ~
      spool:     { type: memory }
  </code></pre>
  
  <p>The only other changes I've made were in validation of a few custom fields.</p>
  
  <pre><code>registration:
      form:
          type: acme_user_registration
          validation_groups: [MyRegistration]
      confirmation:
          enabled:    true
  </code></pre>
  
  <p>Any ideas what's going on?</p>
  
  <p>Thanks.</p>