I recently did some tweaks and ran across a very interesting issue when hardening PHP using SuHosin. All the WordPress sites stopped working. It seems WordPress needs 256MB in order to work, and by default SuHosin does not specify an upper memory limit.
After a recent upgrade to WordPress 3.5 on a multisite installation, I received an error when using “Update Network” through the GUI. I did some searching via WordPress forums and Google, but did not find anything specific to MU usage.