Full Idea
The procedure for generating Gettier cases involves 'double luck': an instance of good luck cancels out an instance of bad luck.
Gist of Idea
Gettier cases arise when good luck cancels out bad luck
Source
Linda Trinkaus Zagzebski (Virtues of the Mind [1996], III 3.2)
Book Reference
Zagzebski,Linda: 'Virtues of the Mind' [CUP 1996], p.295
A Reaction
You can end up with the right answer in arithmetic if you make two mistakes rather than one. I'm picturing a life of one blundering error after another, which to an outsider seems to be going serenely well.