I don't know whether any body faced this problem or not but I have seen that whenever I give "!" (Exclamation mark) as value in any input field which is defined as mandatory in the selection screen, SAP totally ignores it and gives error message. Whereas if you provide any other special character as input SAP recognizes it.

For example check this program: 

REPORT  Z_CHAR_DEMO  .

parameters: p_char(1) type c obligatory lower case.

start-of-selection.
  write: /'Your have entered: ', p_char. 

Here I have a mandatory parameter of character type with length 1. If you give any value except "!"/ " "(blank) it will display output. 

But for "!", it will display error message.

I have been told by an R/2 guy that this problem has a historical reason, in R/2 exclamation mark ("!") is considered as space and R/3 is inherited from R/2 so R/3 still considers "!" as space though it recognizes blank as well. But I don't get any chance to validate his version.