Even if it is not recomended, mixing or shorting wires in the RS232 will not cause permanent damage, in most of the cases. I think you should be carefull especially if your device is powered from a switching mode power supply. If your power supply and/or PC's power supply is accidentaly disconected from earth, high voltages may occur between the ground of your device and the PC ground. When you plug the connection into the serial port you may burn something. Before plugging, check the AC voltage between those two grounds. It should be near 0.
/pisoiu