The Ouija board has been used by thousands of people for spirit communication and is very similar to automatic writing, the only difference between the two being the absence of the board itself in automatic writing. Both forms of communication are very dangerous; as are séances, because they are usually uncontrolled forms of communications. In other words, the individuals that use these channels are usually novices and are unaware of the possible dangers that await!
A controlled situation would be where a psychic, medium or clairvoyant is present. This way the medium could sense if any dangers are present and close off the communication before any harm was done.
The Ouija board is just a piece of compressed wood, sold at virtually all toy stores and occult supply and book stores. Ouija is a combination of two words: "oui" and "ja" which mean "yes" in French and German respectively. The board itself is not dangerous but the form of communication that you are attempting often is.
Most often the spirits whom are contacted through the Ouija are those whom reside on "the lower astral plane". These spirits are often very confused and may have died a violent or sudden death; murder, suicide, etc. Therefore, many violent, negative and potentially dangerous conditions are present to those using the board. Often times several spirits will attempt to come through at the same time but the real danger lies when you ask for physical proof of their existence! You might say, "Well, if you're really a spirit, then put out this light or move that object!" What you have just done is simple, you have "opened a doorway" and allowed them to enter into the physical world and future problems can and often do arise.
I would strongly advise against the use of the Ouija, automatic writing or séances. If you don't happen to be lucky or unlucky enough (depends on how you look at it) to contact any ghosts, then if nothing else, you could become emotionally attached or dependent on the board's advice. This can be psychologically adverse on the psyche as you will attempt to get more and more information out of the board and you could actually just be using the board as an avenue to the inside of your own mind without realizing it. In other words, you could be asking questions of the board and then your subconscious mind would be using the planchette to answer your own questions! This often happens to those people who use the board by themselves.
I believe the following cases are true examples of what could happen to those who continue to use the board or become obsessed or possessed!