Latest problem that ate too much of my life already (took me a night to get through) was importing some content from a Plone 2.0 site to a new instance of Plone 2.5. The old content was created using an old version of AT (probably 1.2). To prepare the migration I've created a new content type using ArchGenXML that mimicked the old product name, classes and fields and exported the content from the old Zope instance as a .zexp file.
The process should have been pretty much straightforward, but I hit some problems: the content types for the objects and fields was broken (it was printed as <plain_text at broken>). I had to write a script (which used an ExternalMethod library that I used to examome objects) that changed the content type to text/structured and text/plain on several fields. Next, there were lots of unicode decoding problems that made impossible cataloguing the content. So I ran again a script to convert that content and finally things started to look right.

I believe the problem with the invalid content type was due to the fact that the mime_types package from MimetypesRegistry was refactored (moved or replaced) since Plone 2.0 was released with Archetypes 1.2.