Desktop version 2016.5.1401d won't let me type a capital "C" in the login name for new entries. Lowercase "c" is okay. "C" does work in universal build.
Editing an existing entry name and using left or right Shift + C brings up the Copy username, Copy password context menu instead of allowing the character.
1Password Version: 2016.5.1401d
Extension Version: Not Provided
OS Version: Not Provided
Sync Type: Not Provided