(Found out while verifying
CRM-1947 for CiviCRM 2.0.)
Create a new custom group with a custom field of three checkboxen, the first two of them checked by default.
If you go to that field's preview, the default checkboxen are not checked (group's preview works ok).
Now, create a profile with this field and preview it. The checkboxen are checked, but there's also an 'Sorry. A non-recoverable error has occurred. is not of the type Integer' error displayed.