CiviEvent: Could not find valid value for id (Drupal 7.6.1)
After doing a lot of trial and error and closely analyzing the complaints we have received, I'm fairly sure that the cause of the problem is: you register one person, then click back on your browser and try to register a second. Your browser doesn't properly refresh the page and somehow the id of the event is lost in the process.

I'm not sure what the best fix is - but I'm fairly certain that is the cause.