The Mandela effect is the situation where people misremember something either due to stupidity or forgetfulness due to stupidity.
It is named for Nelson Mandela, the South African activist, whom many believed was comedian Howie Mandel instead. Those people were convinced that Mandel died in prison despite seeing a hairless version hosting game shows to this day. This is due to false memory, where your brain tricks you into, say, thinking you already didn't eat two or three chocolate bars today. In the case of Mandel, it was also a case of wishful thinking since many had tired of him inflating a rubber glove on his head and letting it fly off to God-knows-where. This misperception was so strong the 14 US states made inflating a rubber glove on your head punishable by death, if I remember correctly.
False memory is more common than you think. After all, how many times have you come home and found your spouse or girlfriend/boyfriend in bed with your other spouse or even an aardvark? This happens every week if you're like me, a typical run-of-the-mill something-or-other. Just act like nothing has happened, eat dinner, watch a little TV, check out GeoCities and go to bed. When you wake up, you and your brain will be refreshed and you will find yourself on a park bench with an aardvark next to you like it always was before.
In extreme cases, brain medication may be needed. This might cause additional problems, like remembering who your doctor actually is and what planet he/she comes from. Paranoia may result, making you wonder if your doctor really gave you the right meds and if they are really looking out for your best interests. Just remember that the answer is "no", proven by all the street people that know this to be so.
Just keep in mind that the FSB Russian intelligence apparatus has nothing to do with the Mandela effect. I mean, isn't it unthinkable that a hotel owner with a bad attitude could become President of the United States? It just didn't happen. Just remember that when I snap my fingers, you will awaken completely refreshed and recall nothing that was said here. *snap*
So, don't forget to send me that check for the $500 you still owe me, okay?