A month ago, a client reported a problem with how our software app (for mobile sales order entry) was exporting pricing data to some users. After an hour-long conference call between myself, my colleague and three people at the client's office, we had agreed on a new approach (devised mainly by me) that would involve changing the folder structure on the server, updating software and configuration files on the affected mobile devices, and quite a bit of testing and deployment planning. In all, it would involve hours and hours of work by many people.
Then, I had a Kekulé moment. Late that night, after I had completed the first step (changes to the export software), just as I was dropping off to sleep, I thought of a much simpler change to the SQL query at the heart of the problem that fixed it completely without any of the other planned changes. Five minutes of work instead of many hours. It was a little embarrassing that I didn't see it at the outset, but it was one of those things that is more obvious in hindsight. (The solution was creating an additional  join between two tables that were already involved in outer joins to the main pricing table).