Incorrect vertex coordinates outside of viewport

Some vertices falling outside of the viewport end up extremely far away, distorting polygons on the edge of the screen. This happens in most games, but is particularly visible on the ground of Crazy Taxi, or of the Dead or Alive 2 title screen.

This isn't a rendering problem, they are sent corrupted to the renderer (very high or very negative x/y coordinates, often z=3.3333333 (1/depth), at least in Crazy Taxi. I checked many, many things, and still have no clue what's wrong. Latest hypothesis was a FPU issue, but I found none. If this is indeed a FPU issue, that might also explain some weird geometry in Soulcalibur (Yoshimisu's and Maxi's legs, some other erratically disappearing limbs...).

Senryoku / Deecy

Incorrect vertex coordinates outside of viewport #13