This was found when debugging https://code.google.com/p/chromium/issues/detail?id=111470, which was an example of a cubic curve of width less than a pixel.

1. Implement/setup you favorite way to detect when pixels are blended due to a hairline curve (I have forgotten the method that I instrumented) 
2. Track which x,y locations are written while painting the curve
3. Note that almost all pixels are touched twice, and the last pixel is touched 4 times.

Ideally, each pixel would be touched just once for a single curve.

This effect may only have been seen due to the cubic being broken down into 1-pixel length lines, or maybe even smaller lines that resulted in two 1-pixel lines in the same spot after rounding. I'm not sure.