Today I encountered and diagnosed a bug, confirmed as 136221, a bug in a Kontact Summary widget which displays upcoming events. Apparently it mishandles events that reoccur, and actually saves the error in the file, ruining the event data.
A patch exists, and is included in KDE 3.5.6. The patch works with 3.5.5, and can be compiled into Kontact on Debian Etch. I am currently in the process of doing so, and will post the exact process and resulting binaries as soon as I confirm that they work.
Update: fix works perfectly. Details on this page.