1- open a new Calc document
    2- enter the value 1,234 into a cell and validate
=> you will see something like "1,23" appear in the cell

    3- Change the formatting of the cell to something like: Standard" meters"
=> you will see something like "1,23 meters" appear in the cell

    4- use the "Add Decimal Place" button
=> the unit of the cell is lost and you will see something like "1,2" in the
cell instead of "1,234 meters", which would be the expected result.

Note 1: it works if you use the money formating instead of a unit
Note 2: consequently, it also works if you use a formating similar to the money
formatting, like this one: # ##0,000 [$m3-40C];[RED]-# ##0,000 [$m3-40C]
Note 3: a discussion about this problem is available on the french forum:
http://user.services.openoffice.org/fr/forum/viewtopic.php?f=4&t=13483&p=75488