Like the summary says, create custom field, select a data type of "alphanumeric" and an input field type = "Multi-Select". In Multiple Choice Options try entering text to the "Value" field that contains special characters. The field is rejected with the message "Please enter a valid value for this field."
I haven't tried extensive variations but the dash "-" and the slash "/" are consistent offenders.
I've got 1.7 installed and that's where I first noticed the problem. I have recreated the problem on the demo install at demo.civicrm.org which is 1.8.
Apologies if I've committed a faux pas but this is my very first bug report.