I've been having trouble with Apache 2.0.51, under Linux, effectively disabling user authentication(the AuthType, AuthName, AuthUserFile, Require combo) after one user has authenticated. This has happened under both the prefork MPM and worker MPM. I noticed the problem more quickly under the worker MPM, and the problem was sporadic under the prefork MPM, so I'm suspecting that some variable is being set on a fork, and not on every request.

This is mainly a heads up to koitsu, to look for this problem if/when you install 2.0.51. :b

-Xodnizel