Alterations in the timeline happen all of the time according to the Seth Material by Jane Roberts. The difference with what Seth speaks of and this Mandela effect people are now talking about are in how our consciousness deals with it.
The Jane Robert's "Unknown Reality Volume One and Two" point to the past and future being fluid and alterable due to string theory, and yet string theory in a theoretical sense is never mentioned.
Seth states that different realities that agree on enough certain tenants will align and join.
Seth states it's like a river, a large river and this large river will have smaller tributaries flowing off from it making smaller rivers, these smaller rivers will have more tributaries coming off from it in the form of creeks. It is not uncommon for two creeks to meet and merge, or for two small rivers to meet and converge, and or any combination of what has been mentioned.
From what Seth states, this happens ALL the time. And we are not usually aware of it, due to the change being too small, and or our memories having a consensus in terms of what will be remembered.
What is different in terms of the Seth Material and this Mandela effect, is that it seems there is not a consensus decision being made in the collective unconscious. We are having memories of various realities among various groups of people, and this screams of some kind of technological tampering.
Avalon has seen it's fair share of "timeline" threads and for the most part I've never bit. I've always felt that spirit would supersede science in this respect, and that what ever timeline alterations were occurring would have to be okayed on a spiritual level, and thus a consensus of the collective unconscious would take place and we would all agree on reality. But if we are now NOT all agreeing on reality, I'm a little troubled as to how far this could go, and have THEY figured out a way for technologically altering or combining realities thus superseding spiritual laws.
Is this CERN? Project Pegasus? The Montauk Project? The Philadelphia Experiment? Tesla Technology?
For an introduction to the Mandela Effect see the video below.
https://www.youtube.com/watch?v=lIjqZEAIkgE