So learn about bisecting, aka Binary Chop, aka Delta Debugging, aka Divide and Conquer. When you do bisect, you have to start with a ☺ version and a ☠ version. Then you pick a point in between the two, and test to see if it's ☺ or ☠. After you've done that, you've moved the two closer together, so your search space is smaller. When you are debugging, you have your "☠" version automatically, because you've got something that's not working currently. An important test in Vim is to use vim -u NONE (which skips almost all of the startup sequence) to see if the problem reproduces there or not.