I've just been playing around with the business rules, and discovered something odd, I set Email, Direct Phone and Mobile Phone all to be Business Required, then I made a rule for each field saying that if the field contained data the other two fields were no longer mandatory.
On my first test I just entered something in email and sure enough the asterisks disappeared and I could create the record, but on the next time I clicked on Mobile didn't enter a number, then entered a value in Email, then clicked on Direct Phone. These fields then got a red cross next to them because they didn't have a value even though they were no longer showing as mandatory and I couldn't create the record.
This seems to be an issue across Quick Create Forms and regular forms.
Can anyone else replicate this, or have I made a silly mistake somewhere?