March 30, 2013:
The Raspbian distro has recently distributed an upgrade of the Apache webserver, with a modified version of the /etc/apache2/envvars config file. It is important to keep the old version of this file when installing the upgrade with either raspbx-upgrade or apt-get dist-upgrade. Answer the following question with “N”:

Configuration file `/etc/apache2/envvars'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
 What would you like to do about it ?  Your options are:
 Y or I  : install the package maintainer's version
 N or O  : keep your currently-installed version
 D     : show the differences between the versions
 Z     : start a shell to examine the situation
 The default action is to keep your current version.
 *** envvars (Y/I/N/O/D/Z) [default=N] ? N

As a general rule of thumb, if you are unsure what to select on similar questions (new config files are shipped with package upgrades): In almost all cases the right answer is “N” (keeping the current version), unless you know what you are doing.