I ran into this today and thought it might be something worth jotting down to save others the same pain.
When you’re editing a list in Datasheet view in SharePoint 2010 and you’re working with a Calculated column, you can see the formula at the bottom of the view for the cell in which you have your cursor. All well and good. See the screen snip below; the yellow highlighting is mine.
Since it’s a Datasheet view, however, it’s pretty easy to mistakenly type a number into the cell without noticing that it’s a Calculated column. When you do this, you get the prompt below. Again, my highlighting.
Now, here’s where I messed myself up. I didn’t read the message carefully enough and just thought it was telling me that it would take a while to be helpful. In fact, what was happening was that SharePoint was about to replace the formula in the column with a new formula, =50. This meant that every cell then had the value 50 rather than the previous calculation.
It was easy enough to fix, once I figured out what I had done, but some calculations I was doing in a DVWP sure looked crazy for a bit!