I once turned up at a certain UK regional to fly to a JAA meeting in Brussels, having inadvertently forgotten my passport.
The airline staff were great, and got an agreement from UK immigration that they'd be happy to let me back in the next day. However, Brussells (centre of the European superstate!) were quite clear on the point that regardless of what other ID I had, they weren't allowing me in the country!
So, I drove home, got the passport, and made a later flight.
G