Allowing Only One Check Box to be Selected
From SmartWiki
To prevent a user from selecting more than one Check Box for the associated Custom Field, add this code to the HTML Tag section of the Custom Field:
onclick="onlyonechecked(this)"
NOTE: It is normally better to use a Combo Box custom field with Show as Radio Buttons selected
This can be used for:
The user will not be able to select more than a single check box with this code in place.
For a Select One - Combo Box field, the "Show as Radio Buttons" option can be selected to display the field selections as "radio buttons."
Note
- If using onlyonechecked on a Select Many - Check Box field and you also want to Show/Hide Header you should use _showhideheader instead of _showhideheadercheckbox