validate custom data to prevent submission errors

http://manage.dimagi.com/default.asp?142187
We're using a slug field for key validation now, which should work to prevent this once that gets deployed.

Only way I can think of offhand to circumvent that is to add custom user data which doesn't match your domain's model (which is still possible via bulk upload, although it has to be intentional - "uncategorized-data: my <non-valid> name")