"What has been seen cannot be unseen". Slang.
For citations see: http://en.wiktionary.org/wiki/Citations:what_has_been_seen_cannot_be_unseen
e.g.
At Dachau he was witness to real rather than abstract suffering; what has been seen cannot be unseen, nor can it be rationalized.
Brad Prager, "Suffering and Sympathy in Volker Schlöndorff's Der neunte Tag and Dennis Gansel's NaPolA", in Screening War: Perspectives on German Suffering (eds. Paul Cooke & Marc Silberman), Camden House (2010), ISBN 9781571134370, page 196
Also, although 'unforgettable' by itself has a positive connotation, 'unforgettably' doesn't necessarily. E.g. The incident was was unforgettably traumatic