kwin it is!

"Next time" for this problem arrived rather quickly. kwin started spewing errors almost nonstop, like

kwin: (0x40d6b527): RenderBadPicture (invalid Picture parameter)
kwin: (0xcd613f): BadMatch (invalid parameter attributes)
kwin: (0x40d6b526): RenderBadPicture (invalid Picture parameter)
kwin: X_FreePixmap(0x40d6b527): BadPixmap (invalid Pixmap parameter)
kwin: (0xcd614d): BadMatch (invalid parameter attributes)
kwin: (0x40d6b526): RenderBadPicture (invalid Picture parameter)
kwin: X_PutImage(0x40d6b532): BadMatch (invalid parameter attributes)
kwin: X_FreeGC(0x40d6b520): BadGC (invalid GC parameter)
kwin: (0xcd639c): BadMatch (invalid parameter attributes)
kwin: (0x40d6b520): RenderBadPicture (invalid Picture parameter)
kwin: (0xcd6414): BadMatch (invalid parameter attributes)
kwin: (0x40d6b52e): RenderBadPicture (invalid Picture parameter)
kwin: (0x40d6b520): BadDrawable (invalid Pixmap or Window parameter)
kwin: (0xcd641e): RenderBadPicture (invalid Picture parameter)
kwin: (0xcd641e): RenderBadPicture (invalid Picture parameter)
kwin: X_FreePixmap(0x40d6b520): BadPixmap (invalid Pixmap parameter)
kwin: X_FreeGC(0x40d6ad33): BadGC (invalid GC parameter)
kwin: X_FreeGC(0x40d6ad38): BadGC (invalid GC parameter)

And then the garbage window decorations reappeared. Killed and started kwin and the problem goes away!

Info:
% kwin -v
Qt: 3.3.8b
KDE: 3.5.10
KWin: 3.0

Kubuntu package: 4:3.5.10-0ubuntu1~hardy2

The memory use monitoring was inconclusive because I started it only a couple of hours before this "reset". Continuing to gather statistics.