Here's another one I've been thinking about lately. 
We have concluded in earlier discussions : 'natural primary keys are bad, artificial primary keys are good.'
Working with Hibernate earlier I have seen that Hibernate default creates one sequence for all tables. At first I was puzzled by this, why would you do this. But later I saw the advantage that it makes linking parents and children fool proof. Because no tables have the same primary key value, accidentally linking a parent with a table that is not a child gives no results.

Does anyone see any downsides to this approach. I only see one : you cannot have more than 999999999999999999999999999 records in your database.