In 1922 Otto Stern and Walther Gerlach demonstrated the directional quantization of angular momentum by sending silver atoms through an inhomogeneous magnetic field. Silver has only one electron in the valence shell, so the orbital angular momentum vanishes and only the electron spin contributes to the total angular momentum of the atom. Depending on the orientation of the spin relative to the magnetic field, the atom takes one out of two trajectories, leading to a discrete splitting of the beam after it passed the magnetic field. Classically, one would expect a smooth distribution. This experiment, conducted in Frankfurt am Main, is known today as the Stern-Gerlach experiment, and was one of the milestones on the way to quantum mechanics.
But it was not just the ingenuity of the experimenters that lead to success since originally Stern and Gerlach couldn't see anything on the screen that should be showing two discrete lines. Dudley Herschbach, who won the Nobel prize for Chemistry in 1986, retold Stern's description of the discovery as follows:
"After venting to release the vacuum, Gerlach removed the detector flange. But he could see no trace of the silver atom beam and handed the flange to me [Stern]. With Gerlach looking over my shoulder as I peered closely at the plate, we were surprised to see gradually emerge the trace of the beam... Finally we realized what [had happened]. I was then the equivalent of an assistant professor. My salary was too low to afford good cigars, so I smoked bad cigars. These had a lot of sulfur in them, so my breath on the plate turned the silver into silver sulfide, which is jet black, so easily visible. It was like developing a photographic film."
The complete story of Stern and Gerlach's experiment can be found in Physics Today 56 (December 2003) Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics (pdf), by Bretislav Friedrich and Dudley Herschbach. They also went on to test the plausibility of this story and repeated the original experiment at its 80st anniversary. The found that bad breath alone wouldn't do the trick, but that more likely Stern was actually puffing on a cigar when Gerlach handed him the invisible result.