I have had that error happen on occasion. Without know more about your autoloader or your setup, you can fix the problem with your mocks by using disableAutoload on the classes.
Using the MockBuilder interface:
$mock = $this->getMockBuilder('SomeClass')->disableAutoload()->getMock();
Or
$mock = $this->getMock('SomeClass', array(), null, null, true, true, true)
disables Autoload ----^
http://phpunit.de/manual/current/en/test-doubles.html#test-doubles.stubs.examples.StubTest.php
By default, all methods of the given class are replaced with a test
double that just returns NULL unless a return value is configured
using will($this->returnValue()), for instance.
When the second (optional) parameter is provided, only the methods
whose names are in the array are replaced with a configurable test
double. The behavior of the other methods is not changed. Providing
NULL as the parameter means that no methods will be replaced.
The third (optional) parameter may hold a parameter array that is
passed to the original class' constructor (which is not replaced with
a dummy implementation by default).
The fourth (optional) parameter can be used to specify a class name
for the generated test double class.
The fifth (optional) parameter can be used to disable the call to the
original class' constructor.
The sixth (optional) parameter can be used to disable the call to the
original class' clone constructor.
The seventh (optional) parameter can be used to disable __autoload()
during the generation of the test double class.