What is the shortest program that can replicate this bug?

It belongs to the story that more often than not, I'm thrown into the middle of someone else's development cycle when they get an error message they can't figure out, so fully appreciating every corner of their 100.000 line application is out of the question.

I can either start building a minimal scaffolding to reproduce the issue from scratch, or modify the genuine article to dump its state just prior to the problem and work from the snapshot. Either way, the attempt to produce two different programs that cause the same failure usually reveals what it is.

In the rare cases when that doesn't help, the questions diversify too much to fit in a short answer, but this question is almost always what I start from. It's an expensive strategy, though, I wouldn't use it for bugs of my own creation.