Version: unknown (using 4.3.00 (KDE 4.3.0), Mandriva Linux release 2010.0 (Cooker) for i586) Compiler: gcc OS: Linux (i686) release 2.6.31-desktop-0.rc5.2mnb I had an existing event in my calendar that was not categorised. I edited the event and changed the category to holiday. When I clicked 'apply' the event was deleted! I tried this with several other events and it only seems to happen with 'all day' events. It also seems to be related to whether or not the category has custom colour associated with it, but I cannot confirm this 100%. If the new category has a custom colour then the event is almost always deleted.
I have just discovered that the original 'deleted' events return if you reboot!
I have tested categories extensively in the last weeks, and have been unable to reproduce this bug. This, missing activity for more than six years here and several technology changes since then highly suggest that this bug has been solved somewhere along the way.