Dr Who / Van Gogh

Last night's Dr Who episode featuring a time travelling visit to meet Vincent Van Gogh was a good one (see Impworks review of the episode). The best thing about it for us was that Samwise had just been learning about the "Starry Night" paintings at school, so we'd bought him a book on Van Gogh. Also in the book is the "Church at Auvers". When you look closely you can see a dark smudgy mark in the bottom right corner of one of the windows - as if something has been painted out...?