Why am I getting random fatal errors - "Class" Proxies \ MyEntityProxy "not found"?

I keep getting (what seems to be) random PHP fatal errors complaining:

Fatal error: class 'Proxy \ MyEntityProxy' cannot be found in /vendor/doctrine/lib/Doctrine/ORM/Proxy/ProxyFactory.php on line 94

Usually I just update and everything works. I am using Symfony 2.0.9 (currently upgrading to 2.0.11). I have never seen this error before and am not sure how to fix it.