This has happened on my laptop on every version of KDE4 I have ever tried!
Just before the desktop appears I get the “dung” error sound.
I have no idea what is crashing and don’t really know where to look either.
Sometimes if I logout I get to see one of those “application crashed” boxes, but it goes too fast to see much.
Is there any way I can find out what the error is?
This seems to be distro independent, ie it happens under Ubuntu KDE4 too (different hard drives).
Plus using KDE4 is always like driving a fully loaded 18 wheeler with a Datsun 4 cylinder engine and flat tyres, sluggish, unresponsive, and cumbersome.
I’m hoping this error has something to do with that and I can fix it, as there always seems to be a “knotify4” running (I assume that is the error box thingy!).
Thanks 
Oh yes, machine specs: HP dv6645 laptop, 2g RAM, Nvidia 8400m gs, openSUSE 11rc1 + all recent updates.
Do you believe it?
I was actually able to get an error message from that box!
It says:
e?1034h(no debugging symbols found)
----- snip -----
(no debugging symbols found)
(x10)
----- snip -----
[Thread debugging using libthread_db enabled]
[New Thread 0x7ff529d4b740 (LWP 4566)]
----- snip -----
(no debugging symbols found)
(x50)
----- snip -----
[KCrash handler]
#5 0x00007ff52ce5e5c5 in raise () from /lib64/libc.so.6
#6 0x00007ff52ce5fbb3 in abort () from /lib64/libc.so.6
#7 0x00007ff52ce9c3a8 in ?? () from /lib64/libc.so.6
#8 0x00007ff52cea1af8 in ?? () from /lib64/libc.so.6
#9 0x00007ff52cea3508 in ?? () from /lib64/libc.so.6
#10 0x00007ff52cea36e6 in free () from /lib64/libc.so.6
#11 0x00007ff53161df3d in ?? () from /usr/lib64/libQtCore.so.4
#12 0x00007ff531627809 in ?? () from /usr/lib64/libQtCore.so.4
#13 0x00007ff52ce6126d in exit () from /lib64/libc.so.6
#14 0x00007ff52ce4a43d in __libc_start_main () from /lib64/libc.so.6
#15 0x0000000000400879 in _start ()
#0 0x00007ff52cece261 in nanosleep () from /lib64/libc.so.6
Hope that helps