Version:           unknown (using KDE 3.2.90 (3.3 alpha1, CVS >= 20040522), compiled sources)
Compiler:          gcc version 3.3.2 20031218 (Gentoo Linux 3.3.2-r5, propolice-3.3-7)
OS:                Linux (i686) release 2.6.3

The bug is reproducable with some apps using KSystemTray, e.g. the wallet manager or kmix.
1) Restore the window
2) Shade it
3) Click on the tray icon to hide it.
The bug doesn't occur when using "Minimize" from the system tray context menu.

KWin output:
...
kwin: User timestamp, final:'ID:35651589;WMCLASS:kwalletmanager:kwalletmanager;Caption:KDE Wallet Manager':18610433
kwin: Activation, compared:'ID:35651589;WMCLASS:kwalletmanager:kwalletmanager;Caption:KDE Wallet Manager':18610433:18595395:true
kwin: Raising: Belongs to active application
kwin: Activation: Belongs to active application
kwin: h:259:t:25:b:4
kwin: /home/sandro/src/kde/kdebase/kwin/geometry.cpp:1408: void KWinInternal::Client::plainResize(int, int, KWinInternal::ForceGeometry_t): Assertion `false' failed.

Backtrace:
[New Thread 16384 (LWP 26993)]
[KCrash handler]
#4  0x4157a8c1 in kill () from /lib/libc.so.6
#5  0x41406913 in pthread_kill () from /lib/libpthread.so.0
#6  0x41406cab in raise () from /lib/libpthread.so.0
#7  0x4157a4b2 in raise () from /lib/libc.so.6
#8  0x4157bdac in abort () from /lib/libc.so.6
#9  0x41573d59 in __assert_fail () from /lib/libc.so.6
#10 0x41d37a2e in KWinInternal::Client::plainResize(int, int, KWinInternal::ForceGeometry_t) (this=0x81d40b8, w=328, h=259, force=NormalGeometrySet)
    at /home/sandro/src/kde/kdebase/kwin/geometry.cpp:1408
#11 0x41d364f4 in KWinInternal::Client::configureRequest(int, int, int, int, int, int) (this=0xbfffe550, value_mask=15, rx=-512, ry=-512, rw=320, rh=230, 
    gravity=1) at qsize.h:132
#12 0x41d1939e in KWinInternal::Client::configureRequestEvent(XConfigureRequestEvent*) (this=0x81d40b8, e=0xbfffec20)
    at /home/sandro/src/kde/kdebase/kwin/events.cpp:808
#13 0x41d18d45 in KWinInternal::Client::windowEvent(_XEvent*) (this=0x81d40b8, 
    e=0xbfffec20) at /home/sandro/src/kde/kdebase/kwin/events.cpp:555
#14 0x41d17f79 in KWinInternal::Workspace::workspaceEvent(_XEvent*) (
    this=0x81d40b8, e=0xbfffec20)
    at /home/sandro/src/kde/kdebase/kwin/events.cpp:267
#15 0x41d0e68e in KWinInternal::Application::x11EventFilter(_XEvent*) (
    this=0xfffffe00, e=0xbfffec20) at workspace.h:73
#16 0x40bd7f85 in qt_x11EventFilter (ev=0xbfffec20)
    at kernel/qapplication_x11.cpp:382
#17 0x40be14e3 in QApplication::x11ProcessEvent(_XEvent*) (this=0xbfffed80, 
    event=0xbfffec20) at kernel/qapplication_x11.cpp:3269
#18 0x40bfc76b in QEventLoop::processEvents(unsigned) (this=0x80e9820, flags=4)
    at kernel/qeventloop_x11.cpp:192
#19 0x40c6b18d in QEventLoop::enterLoop() (this=0x80e9820)
    at kernel/qeventloop.cpp:198
#20 0x40c6b0a6 in QEventLoop::exec() (this=0x80e9820)
    at kernel/qeventloop.cpp:145
#21 0x40c56e01 in QApplication::exec() (this=0xbfffed80)
    at kernel/qapplication.cpp:2741
#22 0x41d0eaf8 in kdemain (argc=2, argv=0x8061a50)
    at /home/sandro/src/kde/kdebase/kwin/main.cpp:274
#23 0x40929996 in kdeinitmain (argc=-512, argv=0xfffffe00) at kwin_dummy.cpp:2
#24 0x0804cb76 in launch (argc=2, _name=0x8061fdc "kwin", args=0x8061feb "", 
    cwd=0x0, envc=0, envs=0x8061fef "", reset_env=false, tty=0x0, 
    avoid_loops=false, 
    startup_id_str=0xfffffe00 <Address 0xfffffe00 out of bounds>)
    at /home/sandro/src/kde/kdelibs/kinit/kinit.cpp:591
#25 0x0804e1e1 in handle_launcher_request (sock=4)
    at /home/sandro/src/kde/kdelibs/kinit/kinit.cpp:1155
#26 0x0804e7dc in handle_requests (waitForPid=0)
    at /home/sandro/src/kde/kdelibs/kinit/kinit.cpp:1338
#27 0x0804f812 in main (argc=3, argv=0xbffff534, envp=0xfffffe00)
    at /home/sandro/src/kde/kdelibs/kinit/kinit.cpp:1785
#28 0x41566dcc in __libc_start_main () from /lib/libc.so.6