To reproduce: Go to http://www.guardian.co.uk/politics/reality-check-with-polly-curtis/2012/may/01/air-transport-immigration and scroll down a few pages to the embedded spreadsheet. Attempt to move the spreadsheet scrollbars by grabbing with the mouse. This triggers the bug. Clicking to move the scrollbars seems not to trigger the bug.

Reproducible: Always.

Apport produces a crash dump, but no further analysis. gdb says:

Program received signal SIGSEGV, Segmentation fault.
khtml::RenderLayer::calculateRects (this=0x2d04cc0, rootLayer=0x2d04cc0, 
    paintDirtyRect=..., layerBounds=..., backgroundRect=..., 
    foregroundRect=...) at ../../khtml/rendering/render_layer.cpp:1330
1330    ../../khtml/rendering/render_layer.cpp: No such file or directory.

gdb backtrace shows:

#0  khtml::RenderLayer::calculateRects (this=0x2d04cc0, rootLayer=0x2d04cc0, 
    paintDirtyRect=..., layerBounds=..., backgroundRect=..., 
    foregroundRect=...) at ../../khtml/rendering/render_layer.cpp:1330
#1  0x00007fffe4cee0ba in khtml::RenderLayer::nodeAtPointForLayer (
    this=0x2d04cc0, rootLayer=0x2d04cc0, info=..., xMousePos=485, yMousePos=0, 
    hitTestRect=...) at ../../khtml/rendering/render_layer.cpp:1216
#2  0x00007fffe4cee430 in khtml::RenderLayer::nodeAtPoint (this=0x2d04cc0, 
    info=..., x=485, y=0) at ../../khtml/rendering/render_layer.cpp:1193
#3  0x00007fffe4c0bffc in DOM::DocumentImpl::prepareMouseEvent (
    this=0x279f340, readonly=true, _x=485, _y=0, ev=0x7ffffdfff350)
    at ../../khtml/xml/dom_docimpl.cpp:2047
#4  0x00007fffe4b7c16c in KHTMLView::mouseMoveEvent (this=0x2776270, _mouse=
    0x7ffffdffffe0) at ../../khtml/khtmlview.cpp:1336
#5  0x00007ffff443b178 in QWidget::event (this=0x2776270, event=0x7ffffdffffe0)
    at kernel/qwidget.cpp:8347
#6  0x00007ffff47fb3c6 in QFrame::event (this=0x2776270, e=0x7ffffdffffe0)
    at widgets/qframe.cpp:557
#7  0x00007fffe4b801dd in KHTMLView::widgetEvent (this=0x2776270, 
    e=<optimized out>) at ../../khtml/khtmlview.cpp:2209
#8  0x00007fffe4b7f984 in KHTMLView::eventFilter (this=0x2776270, o=0x2771d10, 
    e=0x7ffffdffffe0) at ../../khtml/khtmlview.cpp:2054
#9  0x00007ffff52e5028 in QCoreApplicationPrivate::sendThroughObjectEventFilters (this=<optimized out>, receiver=0x2771d10, event=0x7ffffdffffe0)
    at kernel/qcoreapplication.cpp:986
#10 0x00007ffff43ea85f in notify_helper (e=0x7ffffdffffe0, receiver=0x2771d10, 
    this=0x634af0) at kernel/qapplication.cpp:4555
#11 QApplicationPrivate::notify_helper (this=0x634af0, receiver=0x2771d10, 
    e=0x7ffffdffffe0) at kernel/qapplication.cpp:4531
#12 0x00007ffff43f00bf in QApplication::notify (this=<optimized out>, 
    receiver=0x2771d10, e=0x7ffffdffffe0) at kernel/qapplication.cpp:4102
#13 0x00007ffff5d44b46 in KApplication::notify (this=0x7fffffffd7e0, 
    receiver=0x2771d10, event=0x7ffffdffffe0)
    at ../../kdeui/kernel/kapplication.cpp:311
#14 0x00007ffff52e4e9c in QCoreApplication::notifyInternal (this=
    0x7fffffffd7e0, receiver=0x2771d10, event=0x7ffffdffffe0)
    at kernel/qcoreapplication.cpp:876