In science, many researchers have experienced missed opportunity of big or small discoveries. You may share some of yours here.

1) Masaki Oshikawa told his story in facebook:

2) Tony Zee told me that he have calculated WZW term before WZW, except written it in frequency-momentum space.   In the frequency-momentum space all the beautiful geometric meaning of WZW term is lost.

3) Here is one of my story:

I gave a talk in IBM about topological ground degeneracy and its relation to topological order in 1990. After my talk, I talked to David DiVincenzo and he told me that he was working on "quantum computing" and explain to me what it is. My first reaction is: "WHAT? IBM supports such fundamental research?!" But I did not have the insight to realize that the topological degeneracy that defines topological order can be used as qubits to perform fault tolerant topological quantum computation. But at that time, the notions of  fault tolerant and topological quantum computation had not been invented yet. I might not even know the words "qubit" and "entanglement". (Here is the link of my paper with Niu Qian on topological degeneracy. The paper is ignored for more than 10 years.  http://scholar.google.ca/citations?view_op=view_citation&hl=en&user=jfZtuFwAAAAJ&citation_for_view=jfZtuFwAAAAJ:Y0pCki6q_DkC  )

How do you realize something that the words to describe the thing have not invented yet?

Almost missed opportunity:

I also remembered that people often asked me about if the 1D Haldane phase is an example of topological order? I did not know what to say and was very puzzled. To me  Haldane phase seems quite different from topological order (except many call it topological phase). But what is going on? This question bothered me for 15 years ... ... unitil the paper with Zheng-Cheng arxiv:0903.1069 in which we realized that the Haldane phase has a trivial topological order and behave like a product state, but with an "entanglement" between the symmetry and the short-range quantum entanglement. We invented a name "SPT states" for such kind of states.  This is the start of SPT theory. The later discovered group cohomology describes this "entanglement" between the symmetry and the short-range quantum entanglement.

Lesson learned: Do not forget any of your puzzles