A GUI client application has an MBCS Text field.
Microsoft Windows running under Traditional Chinese locale (Big5 / codepage 950)
The problem is specific to characters which have leading 2 bytes greater than FC
The default codepage.ini file has this entry within the section for "[ 950 ] Chinese Big-5 "
mbcsLeadBytes = 0x81-0xFC|2
If the FC is changed to FE to cover all possible extended character leading bytes the problem is resolved i.e.
mbcsLeadBytes = 0x81-0xFE|2