I was creating a new spreadsheet when I noticed a really strange behavior in Excel. I have used Excel for decades and never noticed anything like this before. I would love to hear any explanation for any logic that might be behind this. Follow the steps below to replicate the strangeness for yourself. Open a brand new spreadsheet and type a simple formula into cell A1 like this: =1+2 Next, copy that cell down to the 3 cells directly below it, so now the four cells A1, A2, A3 and A4 are like this: =1+2 =1+2 =1+2 =1+2 Just to the right of the first cell, type the following formula into cell B1: =A1 Next copy that cell down to the 3 cells directly below it, just as before, so now the formulas in those four cells B1, B2, B3 and B4 are like this: =A1 =A2 =A3 =A4 And now, to see the strange behavior of Excel, type the following into the cell A5: =B1 Boom! You should now see that Excel has taken it upon itself to fill in cell B5 with a formula! Why?!?!? I was pulling my hair out thinking I must have had a corrupted spreadsheet, until I finally isolated the cause. This does not happen if you use less than four rows. It also does not happen if you type in the 1+2 formula manually in each cell, you have to copy them down. I have never seen Excel do anything like this before. Can anyone explain the reasoning here?