As a cardholder of the gold card (Senator status) you have been able to book reward tickets worth more miles than you had in your account. Previously it was possible to spend 160000 miles more than you have. Now, this has changed With the currrent update, you will be able to spend just 50000 miles extra and StarAllicance will charge you EUR 0,024 per mile if you mileage is not positive when you loose your gold status.
See: http://www.miles-and-more.at/meilenvorschuss