In OOo300_m3_EN on WinXP.

To reproduce:
1 Create a new Calc file
2 Select a cell in a sheet at will
3 Data -> Validity -> Allow (e.g. whole numbers) -> data (e.g. less than)
  -> maximum (e.g. 10), namely values which are less than 10 are valid.
4 Switch to Error Alert tab, uncheck "Show error message when invalid values 
are entered", then OK
5 Enter 50 to this cell

Result:
there is no error messages when invalid values are entered if uncheced in 
Error Alert tab

Expectation & Help:
Displays the error message that you enter in the Contents area when invalid 
data is entered in a cell. If unchecked, the message is displayed to prevent 
an invalid entry.