I discovered this while doing -automated +automated testing of upgrades from Debian Lenny to Squeeze. A few packages in Debian still got circular dependencies, and it is often claimed that apt and aptitude should be able to handle this just fine, but some times these dependency loops causes apt to fail.
An example is from todays -upgrade +upgrade of KDE using aptitude. In it, a bug in kdebase-workspace-data causes perl-modules to fail to upgrade. The cause is simple. If a package fail to unpack, then only part of packages with the circular @@ -72,7 +72,7 @@ dependency. Reported both to BTS to try to get someone to look at it.