Dorothy Bishop has an excellent post ‘Ten serendipitous findings in psychology’, in which she lists ten celebrated discoveries which occurred by happy accident.
Each discovery is interesting in itself, but Prof Bishop puts the discoveries in the context of the recent discussion about preregistration (declaring in advance what you are looking for and how you’ll look). Does preregistration hinder serendipity? Absolutely not says Bishop, not least because the context of ‘discovery’ is never a one-off experiment.
Note that, in all cases, having made the initial unexpected observation – either from unstructured exploratory research, or in the course of investigating something else – the researchers went on to shore up the findings with further, hypothesis-driven experiments. What they did not do is to report just the initial observation, embellished with statistics, and then move on, as if the presence of a low p-value guaranteed the truth of the result.
(It’s hard not to read into these comments a criticism of some academic journals which seem happy to publish single experiments reporting surprising findings.)
Bishop’s list contains 3 findings from electrophysiology (recording brain cell activity directly with electrodes), which I think is notable. In these cases neural recording acts in the place of a microscope, allowing fairly direct observation of the system the scientist is investigating at a level of detail hitherto unavailable. It isn’t surprising to me that given a new tool of observation, the prepared mind of the scientists will make serendipitous discoveries. The catch is whether, for the rest of psychology, such observational tools exist. Many psychologists use their intuition to decide where to look, and experiments to test whether their intuition is correct. The important serendipitous discoveries from electrophysiology suggest that measures which are new ways of observing, rather than merely tests of ideas, must also be important for psychological discoveries. Do such observational measures exist?
Mind Hacks 
A plan for research is hard to follow mainly because a plan of any kind is difficult to follow. Along the way, unexpected discoveries find a way to present themselves. In the third chapter of my psychology course, research methods are discussed. Research is a never ending process. There is always something to learn. As technology improvements are made, the search for new information is increasingly expanded. The idea of serendipity is important because the unexpected discovery in the midst of a planned discovery might just be even more substantial than what we were looking for in the first place.