The Mandela Effect, to put it simply, is a phenomenon that occurs when a large group of people seem to misremember an event on a wide scale. Basically, the idea that you have experienced reality differently from what it actually is. It is named after the death of Nelson Mandela, which many people seem to falsely remember as happening in the 1980s when it actually occurred in 2013. Personally, I had also remembered Mandela as dying in the 80s, and was surprised when I heard that it wasn't until much later that he actually did die. So I am experiencing a different reality, or living in a different universe?
There are a lot of different examples of the Mandela Effect. For instance, the famous line in Snow White being remembered as "Mirror, Mirror, on the Wall" when it was actually "Magic Mirror on the Wall", or that the Monopoly Man doesn't actually have a monocle like a lot of people seem to think he does.
I've even experienced weird glitches in the matrix like this myself. The song "All About that Bass" by Meghan Trainor was said to be released in 2014, but I could swear I heard it for the first time at the age of 12 or 13, which would have been much earlier. That might just be me going nuts.
Many theorize that the Mandela effect is evidence of time travel, or of the fact that we are actually living in a multiverse with many different realities. Personally, though, after doing some research on it, I would put it down to bad memory. It seems like an oversimplified explanation, but human memory is actually a pretty complicated thing.
Human memory is a lot more fragile than many people seem to know, and very malleable. False memories are frighteningly easy to put in people's heads. In one psychological study, adult participants were asked by family members if they remembered being abducted by a man at the mall at the age of 8. In each case, no such event had actually occurred, but participants were told details of the "event". Many participants, of their own accord, claimed to remember the event, even somehow coming up with details of their own, such as the color of the shirt the man who "abducted" them was wearing. Even after being told that this event had not actually occurred, participants continued to insist that it had. False memories were surprisingly easy to implant.
So what does this prove?
Humans have a way of "filling in the gaps" so to speak, especially when it comes to memory. Essentially, we will remember things in the way that makes the most sense for us based on our experiences with the world so far. Even in the present, our brain tends to omit and process information in ways that make sense for us. We are also a very social species susceptible to interpreting information in the way that other people suggest we should.
Perhaps that is why the famous "Luke, I am your father" line from Star Wars is actually "No, I am your father". The word "Luke" makes the quote much easier to identify as being from Star Wars, which is why perhaps people who quote it subconsciously add in that word incorrectly.
People may remember "Froot Loops" as being spelled "Fruit Loops" because even though it was spelled "Froot Loops" on the box, it was spelled "Fruit Loops" in news articles or other promotional material. Things were simply spelled differently based on context, or perhaps may have been spelled incorrectly by accident, leading to the confusion.
While I tend to believe that Occam's razor is true, and the simplest explanation is often the best, I definitely can't say for sure why the Mandela Effect is a thing. So it could be after all that there is multiple universes, or that people have gone back in time to create the inconsistencies we see now.