There may be a few minor issues here and there, but my experience, in nearly 13 years of use of the i386 port, seven years of use of the amd64 port and about four years of use of the mips port, has been very good, with just one breakage, of the package CUPS, on my workstation.
Not so with Wheezy or Debian 7.0 which was released on May 4. One package, Dovecot, a secure IMAP and pop3 server, fails during post-installation. There were so many changes from version 1.2 to 2.0 that it merited at least a mention in the release notes.
The upgrade from Squeeze (the previous stable version) to Wheezy breaks the mail system and one has to manually fix things in order to get Dovecot working again.
It's not as if the problems with Dovecot were unknown. On April 26, user Dominik George filed a bug report in the Debian bug tracking system. He wrote: "The dovecot packages have a new major release in wheezy (Dovecot 1 vs. Dovecot 2). Almost nothing remained the same - the maintainer as well as upstream have chosen to make the two versions heavily incompatible.
"An automated upgrade from the squeeze package to the wheezy package is not possible (right now, the wheezy dovecot package fails in postinst when upgrading from squeeze and also on a fresh installation, but this is another issue). The upgrade needs a lot of manual work to convert the configuration to the new format and also parameters for integration with MTAs changed.
"If the release team wants to add a section about that in the release notes, please notify me and I will happily create one."
Adam D. Barratt, a member of the Debian release team, responded: "http://wiki2.dovecot.org/Upgrading/2.0 suggests that a lot of configuration should still work, albeit with noisy warnings. I'm not sure how well their suggested upgrade approach works with Debian's configuration, admittedly.
"In any case, if you'd like to draft some text then I'd be happy to look at including it."
(The link he referred to gives an incorrect assessment of the upgrade, at least as far as Debian goes, saying, "A lot of settings have changed. Dovecot v2.0 can still use most of the v1.x configuration files, but it logs a lot of warnings at startup. A quick and easy way to convert your old config file to v2.0 format is: <command> This command logs a warning about each obsolete setting it converts to the new format. You can simply ignore all the warnings in most cases.")
Another Debian developer, Christian Perrier, added: "I think you don't need an ACK from the release team: please create such section. Dovecot is 'important' enough to deserve this (and, anyway, whatever contribution to release notes for non-smooth package upgrades is certainly welcomed)."
There the matter ended. Nothing regarding the Dovecot problems was included in the release notes that accompanied Wheezy.
Perrier commented on his blog, regarding the upgrade he carried out on his own server: "Dovecot was more tricky and my mail server temporarily stopped working for my laptop to grab mail from it. OK, admittedly, I should have read NEWS.Debian that was explaining all problems one might have, particularly problems related to SSL certificates and the use of the 'mail' group. But, indeed, that should have deserved a note in the release notes. After all, we're talking there about an obscure php5-suhosin package, right?"
The php5-suhosin package is one that hardens PHP; it has been removed in Wheezy but it was mentioned in the release notes, as was a way to ensure that no error messages were generated by its absence. It is not a show-stopper by any measurement. The problems with Dovecot certainly are a big spoke in the wheel..
The NEWS.Debian file Perrier refers to does enumerate a number of problems and also refers to another file, README.Debian, which is even more detailed. But these files are accessible on a system only after install. By the time that happens, one already has a broken system. Reading at that point does tend to cause more annoyance than illumination.