Clever Hans Effect
Clever Hans was a horse that seemed to be able to perform arithmetic and solve puzzles. However, after a formal 1907 investigation, it was shown that Hans was actually picking up on and responding to subtle audience cues rather than solving problems.
The Clever Hans Effect is important because it exposes how some of our involuntary reactions influence or cue others. This is why there are occasionally problems with police lineups or interrogation of children. It is also why use of double-blind experiments is encouraged.