the Diderot Effect--for a phenomenon I'd observed but never knew had a label. I felt a similar sense of ah-ha elation when I stumbled upon what is known as the Baader-Meinhof Phenomenon.
The Baader-Meinhof Group was the original name of the Red Army Faction, a West German far-left militant group responsible for a number of terrorist acts in the 1970s especially. (Fun fact: in my previous life as an academic, I once edited a friend's master's thesis on post-World War II German far left groups.) But they're not very well known these days, which is how their name came to be attached to a phenomenon also known (less colorfully) as "frequency illusion."
So how did this phenomenon end up with the name "Baader-Meinhof phenomenon"? Evidently back in the 90s, some online commenter at the St. Paul Pioneer Press came up with the label after hearing about the B-M Gang twice in one day. From there it spread.
And now we can all watch to see how many times the term suddenly appears in our lives.