The Render extension tries to draw triangles by effectively sorting the        
three points by their Y coordinates, and drawing two degenerate trapezoids 
that meet at the horizontal line passing through the middle point.  
(see miRasterizeTriangle() in xserver/render/mitri.c) 
The tricky part of this technique is figuring out which edge of the triangle is 
the left edge of the trapezoid and which is the right. The current code does 
this by comparing the X coordinates of the two non-top points, but this doesn't 
work in general: there are cases where the non-top points have the same X 
coordinate, but which edge of the triangle is left and which is right depends 
on the X coordinate of the top point. I think the "two cases" described in 
a comment should really be whether the three points, in top-down order, go 
around the triangle clockwise or counterclockwise. (Which could be determined 
for instance by looking at the sign of cross-product of two edge vectors of    
the triangle).

I'll attach a C program that demonstrates this bug (for me using Xsdl from
last night's CVS, but I think the problem is hardware independent). The 
program's expected behavior is to draw two symmetric triangles (making a
downward-pointing chevron), but only the right triangle draws correctly;
the left one fills in only an area to the left of what should be the left edge
of the top half (trapezoid) of the triangle.

(Also reported to XFree86 as their bug #1275)