Science has published an elegant posthumous article by Daniel E. Koshland, Jr. entitled The Cha-Cha-Cha Theory of Scientific Discovery … representing the 3 categories of discovery: Charge, Challenge, and Chance. In brief:
“‘Charge’ discoveries solve problems that are quite obvious … ‘Challenge’ discoveries are a response to an accumulation of facts or concepts that are unexplained by or incongruous with scientific theories of the time … ‘Chance’ discoveries are those that are often called serendipitous and which Louis Pasteur felt favored ‘the prepared mind.'”
I especially like the image of the “prepared mind”, which recognizes meaning even in “failed” experiments rather than demanding a home-run each time out.