"[What has been seen cannot be unseen](http://knowyourmeme.com/memes/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][1]


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Also, although 'unforgettable' by itself has a positive connotation, 'unforgettably' doesn't necessarily. E.g. *The incident was was unforgettably traumatic*

  [1]: http://books.google.co.uk/books?id=m1MdufwL9QgC&pg=PA196&redir_esc=y#v=onepage&q&f=false