Insert a string in a cell of a Calc document
Insert a second line in the same cell by pressing CTRL+Enter, then digit
something for the 2nd raw.
Confirm everything and exit from the cell, pressing Enter or clicking in other cell.

Modify the cell content applying some formatting *to-the-entire-1st-raw*, for
example applying bold, italics and/or some color or other.

Now when you press Enter to exit and confirm the changes you'll get the *entire
cell content* formatted as above.
If you press CTRL+Z (undo) you will have the cell correctly formatted, with only
the first line formatted. But this obviously, this is not a normal expectation.

I far as I can remember this didn't occur with previous versions (I haven't  any
2.4.x version at disposal the moment)