It happened with me 2 years ago when I was travelling with my parents in a train.
It was around 11:30 am when we boarded the train. Passengers were very few and all were asleep. I took the lower berth, dad on the upper and my mom on the next upper berth. The last one was empty.
We closed the curtains and slept.
Around 5 am or a little before, I was awakened by a horrible nightmare. I usually sleep covering myself all over with blanket from head to toe. When I opened my eyes, I saw a shadow of a fat man sitting on my berth on which I was sleeping. It was dark inside except for the dim light coming from the gap between the curtains and I could only see the shadow from my blanket. I thought he might be dad as it was only 3 of us inside and 4th berth was empty. I stared at that fat shadow for 4-5 minutes guessing who was he.
It was nearly 5:30 am. Suddenly my dad's phone rang and he was still on the upper berth and answered in his sleepy voice. I removed the blanket to see who was that person but saw NO ONE. I was quite frightened. It was merely milliseconds between me removing the blanket and shadow being disappeared.
When I told this to my parents, they called it a dream while my friends say it might be an illusion. But I'm sure it wasn't a dream as my uncle called dad that day at 5:30 am and I remember seeing a shadow from my blanket just before I removed it to confirm he was not dad. How he disappeared is still a mystery for me... And it can't be an illusion as I remember his shadowy face, hands and the hat he wore.
Still wonder what was that...!