When does Excel decide to overflow text into adjacent cells, or cut them at the boundary?

For text to overflow beyond the edge of a cell, the following conditions must be true:

• The cell does not have "Wrap Text" turned on
• The cell is not a merged cell
• The cell contains a value that exceeds the width of the cell
• The adjacent cell is empty* and not a merged cell

• The cell has any of the following horizontal alignments:

  • General
  • Left (Indent)
  • Center
  • Right (Indent)
  • Center across selection
    (Right overlaps the cell to the left; center overlaps on both sides.)

• The cell contents are not rotated (i.e. orientation is set to 0°) _{(Excel 2010 only?)}

• The cell contains a text value. Numerical and date values get converted to ####, or to scientific notation, instead of overlapping adjacent empty cells.
• The worksheet does not have "Show Formulas" turned on

I believe these are all the necessary conditions. If I have missed any, please feel free to edit this answer.

_{*In certain circumstances, an adjacent cell can appear to be empty, but not be, in which case the text will not overflow into that cell, because it is not truly empty. For example, if the adjacent cell contains a formula resolving to "", then it is not empty.}

I have experienced this same issue, and it only applies when the value in the cell is all numeric characters (including decimal point), if the value in the cell has any alpha characters in it, it does properly expand into adjacent cells, but when the value is numeric only it does not. This is true even if you change the format of the cells to Text after the number was entered into the cell.

To fix this I had to pre-format the cells to Text in Excel and then paste the data into the text cells using paste special - values.

Another solution is to put a single quote in front of a number and Excel will treat it as text, allowing it to display in the adjacent cell.

In addition to the instances noted in other responses, it should be noted that Excel will always truncate text overflow if the "Show formulas" option (Ctrl + `) is active on the worksheet. If you regularly use the Ctrl + 1 keyboard shortcut to access the "Format Cells" dialog, it's very easy to accidentally activate "Show formulas", and you might not notice it has happened. This is especially true if you don't actually have any formulas on the active sheet.