The act of witnessing something so traumatic or disturbing that it is futilely wished to be unseen. Most commonly experienced on 4chan's /b/ or senior citizen dating websites.
See also: eye bleach
Me: Oh hey, look, some Nigerian dude emailed me a file called BlueGoatseWaffles! Maybe its a picture of blue goats doing people stuff...like cooking waffles!
*35 seconds later*
Me: What has been seen cannot be unseen.