Windows can not create files with one or more of these characters \ / : * ? " < > |
Linux can not create files with character /
Linux: \t within the name may be converted to TAB (I didn't test it)
Spark will not write a chat log of users (receiving or sending, this does not matter) which contain one of these characters. This could also cause trouble when trying to delete chat logs if the characters are not escaped properly.
Windows, Linux, probably Mac