By definition:  A drop-in communicator is an unknown entity who appears unexpectedly and uninvited during a séance. Drop-in communicators have provided fascinating data to psychical researchers to support the contention that mediums do communicate with spirits of the dead and are not merely manifesting secondary personalities. The potential importance of drop-ins was recognized as early as 1874 by William Stanton Moses an English medium, a d a few years later by early English psychical researchers such as Frederic W.H. Meyers, a founder of the Society for Psychical Research. Drop-ins  are unknown to both mediums and sitters. Thus they are evidence that contradicts the theory of Super Psi, which holds that mediums obtain their information from clairvoyance of published sources and telepathy of facts from the minds of sitters, not from discarnate entities. In the best cases, drop-ins provide information that has never been in print in a public source but that is known to, and can be verified by, a small circle of family or friends … some drop-ins drop out as quickly as they appear, showing up once or twice without making their motive clear. Most cases are inconclusive. – Rosemary Ellen Guiley, Ghosts and Spirits

The following example, case in point, is from The Angry Communicator by Michael Prescott.

Alan Gauld, a longtime and highly respected parapsychological researcher, spent some time working with a home circle in Cambridgeshire, England, in 1959. While with the circle, he learned of a case from 1943 involving a drop-in communicator - someone with no known connection to anyone in the group. Fortunately, detailed records had been kept, including verbatim transcripts of the sessions, which were conducted with a "talking board" (a Ouija board or similar item). 

The communicator in question identified himself as "Adolf Biedebmann," adding, "I always was known and called Gustav." In his first appearance, on January 4, 1943, he was hostile and insulting. The spirit control, "Peter," warned, "A little later I am going to let the eel through.... Humour him. Get to know him. We can then deal with him from here." 

"Gustav" then took over the planchette, insinuating that he had impersonated "Raymond and a Doctor James" (presumably Raymond Lodge and William James) in an earlier session. When told that the group wanted to help him, he replied, "I do not want your help." The group said they were taught to help those who need it. "Wrong teaching," the spirit huffed. "I am not going to be bloody well pally [friendly] with you. Mind your own business. I did not come here to talk to you. Shut up."

He pretended to be a female spirit named Molly or Mollie - his spelling varied - but despite his insistence that they "mind [their] bloody business,"  the group quickly divined that he was male. "I was a man who always kept to himself," he added, inadvertently revealing his sex. "Damn," he commented when his slip was pointed out. 

Religion was "bloody rot," he said. When asked to be reasonable, he replied, "Shut up. Buggar you.... Only Hitler can help. He is the master mind.... I knew Hitler." Claiming his nationality was German, he took credit for a sentence in German that was spelled out by the board in a previous sitting. He lived in London, at a house in Charnwood Lodge. By the end of the session, he seemed calmer and more friendly. "Peter" assured the group that "Gustav" was genuinely sorry for his rude behavior, adding, "And he is German."

Three days later, on January 7, an apologetic "Gustav" showed up, giving his "correct name" (a version of his name given in the earlier session was inaccurate). "I was a rationalist," he said. "A type of religion to follow only the reasoning of one's own mind. It puts a barrier around." That was partly why he had been lonely in the spirit realm. "I was turned seventy when I passed away," he said, going on to explain that his references to Hitler were intended "to hurt." 

On February 4, he made another appearance. "I had my own business," he recalled. "In some remote way I am associated with the Lond[on] University." He said he'd passed over a year ago, and when asked if he was connected with publishing, made reference to the Rationalist Press. 

Gauld, examining the case in 1959, discovered the existence of a Dr. Adolf Gustav Biedermann (only a slight variation on the name Biedebmann, which was presumably misspelled during the session). Miles Edward Allen summarizes:

[He] was a German-born, naturalized citizen of England who lived at Charnwood Lodge on the outskirts of London until he died at the age of 73. He was a fairly wealthy businessman who also worked in the Psychology Department at London University.

Those who knew Biedermann described him to Gauld as an arrogant, obstinate, and aggressive man who, nevertheless, could be a pleasant companion when one got to know him. He seemed to revel in his German heritage and never dropped his accent. One acquaintance portrayed him as "an out-and-out rationalist" who may well have been attached to the idea of Aryan superiority.

He was hostile to claims of telepathy, and to religion. In his will he left money to the Rationalist Press Association. 

The case is strong for several reasons. First, the communicator matches up extremely well with the earthly Biedermann in terms of demeanor, beliefs, and personal history. Second, there was a clear progression from hostility to remorse and friendliness, suggesting the activity of a living mind, not a static bundle of memories. Third, no other communicator at this home circle ever behaved the way "Gustav" did; if the mental influence of the sitters or planchette operators had been responsible for the messages, we might expect to see other cases like this involving the same persons, but there were none. Fourth, the loneliness and confusion of "Gustav" dovetails with statements made through many channelers about the state of mind of spirits who hold a deep antipathy to spiritual values and a strong commitment to the ego. 

The only question I would raise is whether either of the planchette operators could possibly have known of Biedermann during his lifetime and unconsciously recreated his personality, manipulating the planchette either through subtle, unintentional muscular movements or via PK. But this seems doubtful, given the absence of any motive on the part of the sitters to bring "Gustav" to the circle. And it is unlikely that a casual acquaintance could know such details as Biedermann's legacy to the Rationalist Press. 

Source: http://michaelprescott.typepad.com/michael_prescotts_blog/2011/11/an-angry-communicator-.html