Do you have an example? And are you talking about hats/caps, or helmets? There are some caps that have been a part of an event (like the Giro or Zwift Academy) that are only available to people who have done that event. And there is a helmet that is only ‘won’ via the prize wheel at the top of the Alpe. Other than that, though, all other caps and helmets should be awarded through XP level-ups. So, there might be some items (caps, especially) that you may never be able to get (because the event has already passed).