Version:            (using KDE 4.2.0)
OS:                Linux
Installed from:    Ubuntu Packages

ok this is a strange bug... i have a back trace so you guys can see if what i think happend actually happened. 

I installed KPhotoAlbum, which i think is using KDE3 libraries, and as soon as i turned it on Plasma crashed. When plasma reloaded it had some of the widgets with "Unable to load widget", namely the Countdown timer widget and the show dashboard widget. 

backtrace is below:

Application: Plasma Workspace (plasma), signal SIGSEGV
0x00007f624f4966e1 in nanosleep () from /lib/libc.so.6
[Current thread is 0 (LWP 17913)]

Thread 4 (Thread 0x42511950 (LWP 17918)):
#0  0x00007f624d1af2d9 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007f624ff77349 in QWaitCondition::wait () from /usr/lib/libQtCore.so.4
#2  0x00007f625361529c in ?? () from /usr/lib/libQtNetwork.so.4
#3  0x00007f624ff76362 in ?? () from /usr/lib/libQtCore.so.4
#4  0x00007f624d1ab3ea in start_thread () from /lib/libpthread.so.0
#5  0x00007f624f4d4cbd in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 3 (Thread 0x41b79950 (LWP 17919)):
#0  0x00007f624f4cd4b2 in select () from /lib/libc.so.6
#1  0x00007f625003f006 in ?? () from /usr/lib/libQtCore.so.4
#2  0x00007f624ff76362 in ?? () from /usr/lib/libQtCore.so.4
#3  0x00007f624d1ab3ea in start_thread () from /lib/libpthread.so.0
#4  0x00007f624f4d4cbd in clone () from /lib/libc.so.6
#5  0x0000000000000000 in ?? ()

Thread 2 (Thread 0x4101a950 (LWP 17923)):
#0  0x00007f624d1af2d9 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007f624ff77349 in QWaitCondition::wait () from /usr/lib/libQtCore.so.4
#2  0x00007f6236632ef0 in ?? () from /usr/lib/kde4/plasma_wallpaper_image.so
#3  0x00007f624ff76362 in ?? () from /usr/lib/libQtCore.so.4
#4  0x00007f624d1ab3ea in start_thread () from /lib/libpthread.so.0
#5  0x00007f624f4d4cbd in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f6249116730 (LWP 17913)):
[KCrash Handler]
#5  0x00007f6250073ed5 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#6  0x00007f62380fb61f in TaskManager::AbstractGroupingStrategy::groupRemoved () from /usr/lib/libtaskmanager.so.4
#7  0x00007f62380fcbfe in TaskManager::AbstractGroupingStrategy::closeGroup () from /usr/lib/libtaskmanager.so.4
#8  0x00007f6238103998 in ?? () from /usr/lib/libtaskmanager.so.4
#9  0x00007f623810410d in ?? () from /usr/lib/libtaskmanager.so.4
#10 0x00007f6250073134 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#11 0x00007f6238113972 in TaskManager::TaskGroup::itemRemoved () from /usr/lib/libtaskmanager.so.4
#12 0x00007f62380feed8 in TaskManager::GroupManager::remove () from /usr/lib/libtaskmanager.so.4
#13 0x00007f6238100a9b in TaskManager::GroupManager::qt_metacall () from /usr/lib/libtaskmanager.so.4
#14 0x00007f6250073134 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#15 0x00007f6238116d95 in TaskManager::TaskManager::taskRemoved () from /usr/lib/libtaskmanager.so.4
#16 0x00007f623811827e in TaskManager::TaskManager::windowRemoved () from /usr/lib/libtaskmanager.so.4
#17 0x00007f6238119f7f in TaskManager::TaskManager::qt_metacall () from /usr/lib/libtaskmanager.so.4
#18 0x00007f6250073134 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#19 0x00007f6252e7fe62 in KWindowSystem::windowRemoved () from /usr/lib/libkdeui.so.5
#20 0x00007f6252e823ab in ?? () from /usr/lib/libkdeui.so.5
#21 0x00007f6252e91240 in NETRootInfo::update () from /usr/lib/libkdeui.so.5
#22 0x00007f6252e9157b in NETRootInfo::event () from /usr/lib/libkdeui.so.5
#23 0x00007f6252e8267b in ?? () from /usr/lib/libkdeui.so.5
#24 0x00007f6252d559f3 in KApplication::x11EventFilter () from /usr/lib/libkdeui.so.5
#25 0x00007f6254690434 in ?? () from /usr/lib/libkdeinit4_plasma.so
#26 0x00007f62507db7ef in ?? () from /usr/lib/libQtGui.so.4
#27 0x00007f62507ece26 in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
#28 0x00007f62508152c4 in ?? () from /usr/lib/libQtGui.so.4
#29 0x00007f624b7c5d3b in IA__g_main_context_dispatch (context=0x2406a20) at /build/buildd/glib2.0-2.18.2/glib/gmain.c:2144
#30 0x00007f624b7c950d in g_main_context_iterate (context=0x2406a20, block=1, dispatch=1, self=<value optimized out>) at /build/buildd/glib2.0-2.18.2/glib/gmain.c:2778
#31 0x00007f624b7c96cb in IA__g_main_context_iteration (context=0x2406a20, may_block=1) at /build/buildd/glib2.0-2.18.2/glib/gmain.c:2841
#32 0x00007f625008715f in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#33 0x00007f6250814a6f in ?? () from /usr/lib/libQtGui.so.4
#34 0x00007f625005d682 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#35 0x00007f625005d80d in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#36 0x00007f625005fcbd in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
#37 0x00007f625467fd6b in kdemain () from /usr/lib/libkdeinit4_plasma.so
#38 0x00007f624f40c466 in __libc_start_main () from /lib/libc.so.6
#39 0x00000000004007c9 in _start ()