I just had the same problem and lost an hour trying to figure it out.

You should NOT use twice the same field in the same form. We wanted to move a field from one page of a notebook to another. To avoid potential xpath errors it was not replaced from its original place, but only made invisible. And we added it on the page we wanted..

It means that the invisible field was correctly set, but not rendered. When editing the second field, it looked like it was working (field was set, even after clicking on save), but when reloading the page, it disappeared.

Not hard to fix, but a bit confusing to understand why that single field couldnt be edited :)    

in odoo v8 the problem sould be resolved with (if credit_limit is an computed field):

credit_limit = fields.xxx(store=True)