I believe this issue has been here for a long time, but it's very minor and I don't know how easy the fix is.
When the spider's texture is modified to make parts of it not visible the model still obscures the shadow, and turns red when hit, even where there is no texture.
Image proof:
Shadow obstructed: http://i.imgur.com/gAg44iR.png
Entire model red: http://i.imgur.com/lknpGhH.png
Same problem not showing on other mobs: http://i.imgur.com/97oy7Ps.png
No problem when his: http://i.imgur.com/f3krN8X.png
Semitransparent overlay leading me to believe the issue is with the glowy overlay: http://i.imgur.com/HaYlqYk.png