I am a manuscript preacher. For a while I flirted with preaching without a manuscript, but that was simply being a memorized manuscript preacher; I wrote my manuscript, compressed it to notes, and put it to memory. As I have been using PowerPoint to go along with my message, I have stuck to the manuscript (mostly to avoid the "go to the next slide" distraction - I have key phrases highlighted so the tech guys know when to do so without me having to tell them to).
So this Sunday I had already preached my prepared sermon once and was ready to preach it again, but something in my spirit told me that it wasn't the right time to preach it for the second service. So I prayed, "God, if you want me to preach something else, you're going to have to give it to me."
People were praying at the altar and the praise band was leading us in singing, "Praise the Lord, O my soul, praise the Lord" and I opened the pew Bible, figuring I'd start looking for whatever it was that God caught my eye with.
I opened to Psalm 146: Praise the Lord. Praise the Lord, O my soul. I will praise the Lord all my life; I will sing praise to my God as long as I live."
If that wasn't a "sign" I don't know what is. So instead of preaching about evangelism and telling the stories that worked well in the first service, I preached straight through Psalm 146.
Though I have decided while writing a sermon that I've written the wrong one (including doing so on Saturday), I have never ditched my sermon on the spot.
And I did something I never recommend; I simply opened my Bible and went with the exact passage I opened to. Usually I get something like "Then he gave an order: 'Cut the living child in two and give half to one and half to the other.'" (1 Kings 3:25) But in this instance, I really felt God leading me to this particular verse for this particular time.