- What’s new and different?
Father Vaughn always posed that deceptively simple question when asked for help with a new problem.
What he knew, and what we eventually discovered, was that the most recent Thing-That-Changed generally had something to do with what was now broken. Even if the difference didn’t seem related in any way, tracking down its effects was always a highly productive use of your time.
His question applies to non-technical problems, too… especially when you think nothing has changed.
The Smell of Molten Projects in the Morning 
Oh, so true! Even (or, as you say especially, when “it couldn’t be that”)!
I heartily agree with that. I work with computers for a living and this invariable holds true.
A corollary to that is that when you’re making changes, if possible, change one thing at a time.
Or, perhaps, change at most one thing at a time.
I find it educational to run the same test several times and get different results!
And then there’s the series where you change one thing at a time and nothing affects the output. Then you check to see which pin isn’t making contact with its socket…