.....and won! Sometimes this is set in place to delay payment on expensive treatments such as cancer. My husband and I have recently experienced this and succeeded in getting every bill paid.
The trick is to have each doctor fax their first medical notes on you to the insurance company. Plus, have every pathology report faxed as you receive it. This gives them an undeniable path of treatment. I also typed an outline of doctors, dates of treatment, dates of path reports etc for them to follow. They could no longer deny claims as the information was sent prior to the bills!
So, while they will try the ol' pre-existing clause delay tactic, there is a way around it. Keep good notes and enundate them with information.
Hope this helps someone else.
Good luck.