Version:           Plasma Workspace: 0.3 (using KDE 4.3.0)
Compiler:          gcc (Gentoo 4.3.2-r3 p1.6, pie-10.1.5) 4.3.2 
OS:                Linux
Installed from:    Gentoo Packages

Sometimes, the first application in the taskbar disappear (or everything is shifted one position and the first one becomes empty, I'm not sure).
I think it only happens when I switch from one desktop to another.
Once the taskbar is in that state, any modification to it (switching desktop, closing/opening a window) will make plasma crash.
My configuration is 4 virtual desktop, taskbar contains only window on the current desktop.

Here's the backtrace of the crash :
Application: Plasma Workspace (kdeinit4), signal: Segmentation fault
[Current thread is 0 (LWP 8665)]

Thread 2 (Thread 0x7f9d5cb8e950 (LWP 8668)):
#0  0x00007f9d7b76adc9 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007f9d7b9d9315 in QWaitCondition::wait (this=0xf7d570, mutex=0xf7d568, time=18446744073709551615) at thread/qwaitcondition_unix.cpp:87
#2  0x00007f9d7a673cb4 in QHostInfoAgent::run (this=0xf7d550) at kernel/qhostinfo.cpp:260
#3  0x00007f9d7b9d8744 in QThreadPrivate::start (arg=0xf7d550) at thread/qthread_unix.cpp:188
#4  0x00007f9d7b767017 in start_thread () from /lib/libpthread.so.0
#5  0x00007f9d7838548d in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f9d7bf63760 (LWP 8665)):
[KCrash Handler]
#5  Tasks::itemTopMargin (this=0x0) at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/applets/tasks/tasks.h:92
#6  0x00007f9d5fa29d13 in AbstractTaskItem::basicPreferredSize (this=0x1b30910)
    at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/applets/tasks/abstracttaskitem.cpp:99
#7  0x00007f9d5fa23dd0 in TaskItemLayout::layoutItems (this=0x1198ef0) at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/applets/tasks/taskitemlayout.cpp:273
#8  0x00007f9d5fa244b9 in TaskItemLayout::insert (this=0x1198ef0, index=4, item=0x21a54f0)
    at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/applets/tasks/taskitemlayout.cpp:125
#9  0x00007f9d5fa246dc in TaskItemLayout::addTaskItem (this=0x1198ef0, item=0x21a54f0)
    at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/applets/tasks/taskitemlayout.cpp:82
#10 0x00007f9d5fa2f69f in TaskGroupItem::itemAdded (this=0x1198580, groupableItem=0x1746760)
    at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/applets/tasks/taskgroupitem.cpp:477
#11 0x00007f9d5fa301bc in TaskGroupItem::qt_metacall (this=0x1198580, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x7fffb07fa800)
    at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0_build/plasma/applets/tasks/taskgroupitem.moc:113
#12 0x00007f9d7bad0175 in QMetaObject::activate (sender=0x11370a0, from_signal_index=<value optimized out>, to_signal_index=21, argv=0x0) at kernel/qobject.cpp:3101
#13 0x00007f9d5fe74d0f in TaskManager::TaskGroup::itemAdded (this=0x0, _t1=0x1746760)
    at /var/tmp/portage/kde-base/libtaskmanager-4.3.0/work/libtaskmanager-4.3.0_build/libs/taskmanager/taskgroup.moc:139
#14 0x00007f9d5fe765b4 in TaskManager::TaskGroup::add (this=0x11370a0, item=0x1746760) at /var/tmp/portage/kde-base/libtaskmanager-4.3.0/work/libtaskmanager-4.3.0/libs/taskmanager/taskgroup.cpp:138
#15 0x00007f9d5fe651f4 in TaskManager::ProgramGroupingStrategy::handleItem (this=0x221b600, item=0x1746760)
    at /var/tmp/portage/kde-base/libtaskmanager-4.3.0/work/libtaskmanager-4.3.0/libs/taskmanager/strategies/programgroupingstrategy.cpp:154
#16 0x00007f9d5fe5ece4 in TaskManager::GroupManagerPrivate::addTask (this=0x1136650, task={d = 0x7fffb07fa9c0})
    at /var/tmp/portage/kde-base/libtaskmanager-4.3.0/work/libtaskmanager-4.3.0/libs/taskmanager/groupmanager.cpp:288
#17 0x00007f9d5fe5f43a in TaskManager::GroupManagerPrivate::actuallyReloadTasks (this=0x1136650)
    at /var/tmp/portage/kde-base/libtaskmanager-4.3.0/work/libtaskmanager-4.3.0/libs/taskmanager/groupmanager.cpp:155
#18 0x00007f9d5fe5fa91 in TaskManager::GroupManager::qt_metacall (this=0xb58a10, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x7fffb07fab30)
    at /var/tmp/portage/kde-base/libtaskmanager-4.3.0/work/libtaskmanager-4.3.0_build/libs/taskmanager/groupmanager.moc:108
#19 0x00007f9d7bad0175 in QMetaObject::activate (sender=0x11366b8, from_signal_index=<value optimized out>, to_signal_index=4, argv=0x0) at kernel/qobject.cpp:3101
#20 0x00007f9d7baca823 in QObject::event (this=0x11366b8, e=0x7f9d783d3f98) at kernel/qobject.cpp:1066
#21 0x00007f9d78f4d08d in QApplicationPrivate::notify_helper () from /usr/lib64/qt4/libQtGui.so.4
#22 0x00007f9d78f55d0e in QApplication::notify () from /usr/lib64/qt4/libQtGui.so.4
#23 0x00007f9d79f95d31 in KApplication::notify (this=0x6665d0, receiver=0x11366b8, event=0x7fffb07fb1f0)
    at /var/tmp/portage/kde-base/kdelibs-4.3.0/work/kdelibs-4.3.0/kdeui/kernel/kapplication.cpp:302
#24 0x00007f9d7baba79c in QCoreApplication::notifyInternal (this=0x6665d0, receiver=0x11366b8, event=0x7fffb07fb1f0) at kernel/qcoreapplication.cpp:606
#25 0x00007f9d7bae4ab6 in QTimerInfoList::activateTimers (this=0x66ff80) at kernel/qcoreapplication.h:213
#26 0x00007f9d7bae4db8 in QEventDispatcherUNIX::processEvents (this=0x667030, flags={i = -1333807552}) at kernel/qeventdispatcher_unix.cpp:908
#27 0x00007f9d78fdc782 in ?? () from /usr/lib64/qt4/libQtGui.so.4
#28 0x00007f9d7bab93e2 in QEventLoop::processEvents (this=<value optimized out>, flags={i = -1333807424}) at kernel/qeventloop.cpp:149
#29 0x00007f9d7bab957c in QEventLoop::exec (this=0x7fffb07fb700, flags={i = -1333807344}) at kernel/qeventloop.cpp:197
#30 0x00007f9d7babe0fe in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#31 0x00007f9d70614c6b in kdemain (argc=1, argv=0x62d0f0) at /var/tmp/portage/kde-base/plasma-workspace-4.3.0/work/plasma-workspace-4.3.0/plasma/shells/desktop/main.cpp:50
#32 0x000000000040773d in launch (argc=1, _name=0x645008 "/usr/bin/plasma-desktop", args=0x645020 "", cwd=0x0, envc=0, envs=<value optimized out>, reset_env=false, tty=0x0, avoid_loops=false, 
    startup_id_str=0x40b13c "0") at /var/tmp/portage/kde-base/kdelibs-4.3.0/work/kdelibs-4.3.0/kinit/kinit.cpp:676
#33 0x0000000000407dd2 in handle_launcher_request (sock=7, who=<value optimized out>) at /var/tmp/portage/kde-base/kdelibs-4.3.0/work/kdelibs-4.3.0/kinit/kinit.cpp:1168
#34 0x0000000000408542 in handle_requests (waitForPid=0) at /var/tmp/portage/kde-base/kdelibs-4.3.0/work/kdelibs-4.3.0/kinit/kinit.cpp:1361
#35 0x0000000000408a6e in main (argc=2, argv=0x7fffb07fc358, envp=0x7fffb07fc370) at /var/tmp/portage/kde-base/kdelibs-4.3.0/work/kdelibs-4.3.0/kinit/kinit.cpp:1788