The concept of an antimeme has been living rent free in my head (which is quite ironic) ever since I first read this piece back then. Easily my favorite tale from the SCP universe.
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Then you start wondering if you, or anybody, have forgotten anything so important that it would shatter your world.
And would you ever know? Is there any way to ever know?
What if the past changes as much as the future?
What if that's the Mandela effect?
This instance of a poster's existential pondering is not mental illness. It's a valid question. The answer was yes. See repression, then research other cognitive distortions. It's part of building up a healthy meta-cognition.