As Thomas says, character sets are a big problem. Certainly video monitors may use character sets of their own, and you may get odd results if you try to send them codes outside 0x00 to 0x7F, the ASCII range.
Although I work mainly in ASCII, there are still problems because the pound and euro currency symbols (£ and ?) are not in the ASCII character set. And Apple Macs seem to put out weird codes for fancy quote characters etc. http://en.wikipedia.org/wiki/Character_encoding David
