I have five rows of data highlighted with a light-blue fill color. If I enter a number in the cell to the right of one of the rows, in fact, either the first or second column to the right, as soon as I hit Enter, the cell is filled with the light-blue fill color. There is no conditional formatting. I didn't apply any, and I cleared all rules, anyway.

It doesn't seem to do it with all colors. If I put a number in a cell four columns over, it doesn't happen. But if I put numbers in cells in between and work my way over to the fourth column, then it will happen when I enter a number.