Recently, I have come across a number of issues triggered by compatibility issues between Zend, Magento, php 5.4.12, and APC. One, if you cannot print Magento invoices, it’s because of one of the files in the Zend framework.
The other issue people seem to have, is that they cannot work with products in the back end. The simple solution is to disable APC during the time you are editing in the backend. Hopefully, Magento will upgrade the Zend Framework part soon. I’m going to look to see if it is possible to do this before they come out with an update.