So what's incomplete is that there's a testwad using RVZS as a base with sloped lava, and going into the back of the lava wave doesn't hurt - only the front. I need to figure out what's going on there.

https://gfycat.com/UnselfishChillyAuk here's an example of these problems - the damage only works when the lava wave overtakes you, not when you try to overtake the lava (and no, it doesn't matter which way the slope linedefs are facing, so it's not an instance of *rover->topheight being used instead of P_GetSpecialTopZ...)