Actually, this is more or less the default captcha with a custom dictionary, different color scheme, and some CSS.
My captcha-ext.properties is:
cap.border=yes
cap.border.c=0,0,0,0
cap.border.th=1
cap.font.color=255,255,255
cap.noise.c=0,0,0
cap.background.c.to=0,0,0,0
cap.background.c.from=0,0,0,0
cap.image.height=50
cap.image.width=280
Since my original post, I've practically rewritten every class for SimpleCaptcha with our own custom Obfuscation, Distortion, Noise, WordRenderer, WordGenerator - and the only thing I've not changed is the underlying framework. I needed a more hardened CAPTCHA, thus I had to randomize Distortion, Noise, Fonts, Sizing and fix a placement bug in the original distro. The issue happens a lot less frequently since the rewrite, thus, I'm almost convinced it's an issue of poor cleanup.
Another possibility, which I've not pursued yet, is that the headers sent by the captcha image servlet are wrong. The mime type is sent as text/html, which might be related, but not sure yet..
