- Who was at fault?
- It seems that something like a key database table field should have been spotted earlier in the process, who signed off on the beta testing?
- Who wrote the table create statments?
- How long has the person been doing development?
- Who made the work assignment?
- Who wrote the requirements?
As for the tone in which to address them, that is the same tone you wish to be addressed in when you do something that someone else will think is stupid.