I once heard a cautionary tale about the dangers of the observer-expectancy effect. It was at least presented as a true example from the history of science, but I'm having trouble identifying the incident now.
A team of experimental physicists announced they had discovered a previously unknown effect of some sort. The experiment involved some solid sample (a metal or crystal?) of something reasonably available. The setup somehow produced an image related to the sample or its surroundings (maybe a diffraction pattern?). When some additional stimulus or condition was added, the effect was said to cause a specific sort of pattern in the image.
Other groups were interested, but attempts to replicate the experiment were inconsistent at best. The original group explained the experiment is delicate, and invited others to come to their lab to observe it.
The demo went pretty normally: the hosts first showed and explained the apparatus. One researcher set things up for a few "control" runs, and another reported each time the pattern was not visible in the image. Then a few runs were done with the extra condition applied, and the viewer reported each time the pattern was visible.
As the demo concluded, one visiting skeptic revealed they had removed the sample before the measurements started! (My vague memory suggests possibly this skeptic is also known for other things in science or philosophy?)
Does this describe some actual incident, perhaps embellished a bit? If so, what was it?