“The meeting was called to discuss photoreactivation, which had recently been discovered by Kelner in bacteria, and by Dulbecco in phage. It just amazed me that this very striking effect had not been discovered before. Many scientists had irradiated bacteria and phage with ultraviolet light, including Luria, myself, Dulbecco, and so on and so forth, and had measured survival rates. It turns out that if you measure the survival in the presence of daylight, then you get entirely different values than when you measure survival in the dark or in red light. The reason that it hadn’t been discovered was because whoever had done the measurements had done them very carefully under controlled conditions, always the same light. Both Kelner and Dulbecco had done the experiments in a little more sloppy way, sometimes the plates here, and sometimes putting the plates there, sometimes having the water bath near the window, and sometimes not near the window, and then noting that there was something grossly different. So in introducing this little symposium, I said it show the usefulness of limited sloppiness. If you are too sloppy, then you never get reproducible results, and then you can never draw any conclusions; but if you are just a little sloppy, then when you see something startling, you say, ‘Oh, my God, what did I do, what did I do differently this time?'”
Max Delbruck, in a long set of interviews archived at a CalTech oral history project.