1. `copy_cache` in `models.py` recursively copies the caches to environment, 
2. which in
```python
def _compute_related(self, records):
        """ Compute the related field ``self`` on ``records``. """
        others = records.sudo() if self.related_sudo else records
        if records.env != others.env:
            copy_cache(records - records.filtered('id'), others.env)
        values = list(others)
        for name in self.related[:-1]:
            values = [first(value[name]) for value in values]
        for record, value in zip(records, values):
            record[self.name] = value[self.related_field.name]
```
3. copies failing cache values that have been produce previously by this line:

`models.py : def _prefetch_field()`
```python

        try:
            # THIS ONE
            result = records.read([f.name for f in fs], load='_classic_write')
        except AccessError:
            # not all records may be accessible, try with only current record
            result = self.read([f.name for f in fs], load='_classic_write')
```
I checked it's exactly the same `FailedValue` object, which is later accessed without intent (not refreshed) by the fact that the buggy environment is copied to sudo (and not refreshed).


""" return the selection list (pairs (value, label)); labels are
            translated according to context language
        """
        selection = self.selection
        if isinstance(selection, basestring):
            return getattr(env[self.model_name], selection)()
        if callable(selection):
            return selection(env[self.model_name])
        
        if env.lang:
            name = "%s,%s" % (self.model_name, self.name)
            translate = partial(
                env['ir.translation']._get_source, name, 'selection', env.lang)
            return [(value, translate(label) if label else label) for value, label in selection]
        else:
            return selection
    def get_values(self, env):
        """ return a list of the possible values """
        selection = self.selection
        if isinstance(selection, basestring):
            selection = getattr(env[self.model_name], selection)()
        elif callable(selection):
            selection = selection(env[self.model_name])
        return [value for value, _ in selection]
    def convert_to_cache(self, value, record, validate=True):