Saw similar problems when I was doing SVGMorph (some time ago, on SqueakMap) fixed mostly by ensuring the pipeline doesn't get flushed part way through a closed shape.

----- Original message -----
From: Igor Stasenko  <siguctua at gmail.com>
To: The general-purpose Squeak developers list  <squeak-dev at lists.squeakfoundation.org>
Sent: Mon, 24 Mar 2008, 09:35:21 GMT
Subject: [squeak-dev] Bezier filled shape rendering artifacts
Hello,
i spent a few hours to implement new morph (BezierMorph) , partly to
see how balloon draws bezier shapes, partly to be able to edit all
cubic bezier parameters.

What i found that at some specific point it produces a visual
artifacts by infinitely extending a single horizontal lines.

Can't say, is this bug of balloon plugin or it's because of
inaccuracies , which can be eliminated/avoided by ST, without fixing
plugin?
I'm specially making sure that all segment joints having same
coordinates (endpoint  of segment A is equal to starting point of
segment B, which follows A). Also shape is closed.

To reproduce a bug, file in a morph. Put it on desktop, then
right-click -> convert to filled shape.
You can also edit a morph's points (by choosing 'edit shape' in
context menu) and see how horizontal line appears and disappears
depending on vertex position and segments curvature.

Btw, time to time i see same artifacts in hint popups which using
filled curved shapes too.

-- 
Best regards,
Igor Stasenko AKA sig.

<Attachment:>  <BezierMorph.st.gz>