I updated to Magnolia 4.4.3, my frontend configuration is clustered, two public instances.
When the load rises, sometimes this exception below happens. This is particularly problematic because after the problem the instance is completely locked, and shows the magnolia login.
This seems to be caused by modification done in MAGNOLIA-3557, that stores the access count. Doing so for the anonymous user seems to cause concurrent modification. I'll do further investigation, but this is a real blocking problem for me because my instances are failing often.

ERROR info.magnolia.cms.security.SystemUserManager 28.04.2011 10:35:53 – Failed to login as anonymous user
javax.security.auth.login.LoginException: java.lang.RuntimeException: javax.jcr.InvalidItemStateException: 2d78094b-8f7e-4c95-8b1d-22e3dc417c34/{}failedAttempts has been modified externally
at info.magnolia.cms.security.MgnlUser.setFailedLoginAttempts(MgnlUser.java:96)
at info.magnolia.jaas.sp.jcr.JCRAuthenticationModule.matchPassword(JCRAuthenticationModule.java:140)