This happened about 2 years a ago (I was about 13). I had just finished a book about this psychic who had a ghost come to her for help on passing on, the book was based on a true story. Well, since the book dealt with contacting the other side, I wanted to see if it worked.
I was bored so after finishing the book I started concentrating really hard trying to communicate with the other side. I was focusing on the name Aaron (which was the ghost's name in the book, though it had been changed to protect the people involved). And as I was focusing I felt a slap on my wrist. I got freaked (and it broke my concentration) so I immediately stopped and turned on the TV.
The next day I told my mom and she said that it was probably giving me a warning, that I was supposedly opening an unknown portal and whatever slapped me was warning me about that. She also said it could have been my guardian angel. I really don't think it was a ghost because our house is blessed, so it stops "evil spirits" or just spirits from entering, or so we believe.
Another incident that has happened 2 times in the past are knockings on my window. It's a one story house, but it still freaked me out, obviously. We have a dog, and when those incidents happened he didn't bark and it usually barks when someone he doesn't know is near, and it didn't so I'm not sure what it was.