This is a very generic question, but there is a very specific case where this error occurs.

1. If you have 2 or fields that are declared as calculated (doc.openerp.com/6.0/developer/2_5_Objects_Fields_Methods/field_type)
2. If you declare them as multi (i.e. you are using a single function to calculate them.
3. In the return value, you forget to return one of these multi fields.

Basically what happens here is that some underlying functions expects a argument in the result and it is missing.

What does this mean to you? First uninstall any non standard modules that you may have installed and the gradually add them back until you hit the problem. Report back a more specific problem.