My Web sites are mostly running Joomla!, and they were mostly set up originally with Joomla! version 1.5. They survived an upgrade to 2.x and, more recently, to 3.4.
Except for one thing. On this (my personal) Web site, I use the Joomla! WebLinks component, and specifically the SJL AllWebLinks extension that allows a nicely formatted presentation of a set of Web links. OK, this all worked, except that I lost the ability to edit my links. The editor screen, if it came up at all, appeared in plain HTML, no style sheets, no menus, essential elements (such as a button to create a new link) missing.
I tried uninstalling, reinstalling, you name it. Things were progressively getting worse after every try. Ultimately, I happened upon the cause purely by accident: the core WebLinks component of Joomla! was removed in version 3.4. As a result, my upgrade still tried to use the leftover 2.5 version, which had severe compatibility issues.
The solution seemed obvious: uninstall all WebLinks-related components by hand, and then install the (no longer core) standard WebLinks package before reinstalling AllWebLinks. I did this all, and it seemed to have worked (I regained the ability to edit links) except for one minor fly in the ointment: all my links (nearly 500 of them) were gone! As it turns out, the component uninstall/reinstall killed all WebLinks content in the database.
Thank goodness for backups. But even that was less than trivial: in addition to having to restore the _weblinks table, I also had to restore entries in the _categories table (entries for which the extension field had the value com_weblinks). Worse yet, the structure of both tables changed since version 2.5, with the addition of a new field (version).
In the end, I won the battle, but I do feel a little scarred.