False memories
My family brought me up the Eiffel Tower when I was very young but I have no memory of that instance. I don't recall a grand view of the city or waiting in line at the bottom. I don't remember using an elevator or climbing up stairs. I know that I was on the Eiffel Tower and I consider it an achievement somehow. Truth be told, you are one YouTube video away to have a better idea than me about what being on top of Paris feels like.
I sometimes don't remember entire chunks of recent travels either until I see a photograph. It isn't so much a trigger as it is mere proof. Eventually I begin to remember what that photograph looks like and over time, that one single image reshapes my memory of the experience and becomes a foundation for nostalgia.
Now imagine opening an old, private camera folder and one inconspicuous picture, not yours, had been transfered inside. If it's nothing too crazy, if it makes sense and fits the context, certainly you would own the experience without thinking too much about it.
When you're dreaming about past events, this forgery happens by itself. Then you might dream again and again about these inaccurate flashbacks, further blurring the line between what's real and what's imagined. I said dreaming but conscious thought is of course prone to the same distortions.
With the rise of immersive VR technologies and machine learning (deepfakes, chatbots, etc), it might become easier to merge false memories with the real ones. It could be used for fun or as a tool to de-emphasize traumatic experiences or something.