ORA-01722: invalid number
The strange thing with this error is that it may occur sporadic and you can't realy understand why it happens. If you operate on different systems (development, testing, production) you may receive this error on one of the instances and never on the other one. I asked Patrick Wolf about the reasons and he answered that this has to do with the way the cost based optimizer works. If it uses a different plan from the one you would expect, it may find non numeric values in the same column comming from a different collection. In that case the solution is to use the numeric columns in the collection (n0xx) for storing numeric values used in later joins.