Unusual behaviour over serial communication
Knowing the entire setup would help.  For instance, knowing if the RS232 source will drive the Arduino input's dangerously past the Arduino's power rails.

If you have connected a real RS232 signal to the Arduino, the +12 to -12 volt swings could adversely affect the processor.  Causing unexpected behavior.  In such a case you would need a converter like this one.

Over driving inputs is discussed in this Stack Exchange question where the highest scoring answer says:


  If you violate the spec sheet normal operating conditions requirements
  you can expect to experience abnormal operating conditions. These may
  range fro perfectly normal opration through complete maloperation...


That answer also links to this tutorial about logic level conversion. 

Finally, there is this tutorial which specifically addresses converting RS232 to safe TTL level signals appropriate for a micro processor.