When Croupier was first released - in the UK in 1999 - the British Film Institute rescued it from going straight to video. But it had an extremely limited release in just a few cinemas and disappeared. Mike Hodges, the director, later complained in an interview that "....it didn't really have a proper poster..." After its subsequent success in the US it was re-released in the UK in 2001 with a familiar quad poster on display in cinemas.
This poster doesn't look much and the torn corner doesn't help. However, unless someone is sitting on a roll of these, I believe this first release UK Double Crown poster from 1999 is very rare and qualifies as a unicorn.
Mark