Under certain conditions (haven't figured out which conditions yet), the gravestone texture is replaced with what appears to be the texture sheet for the entire game. (I see items from several different mods on it.) It also displays this way when held in the player's hand (creative-spawned). When the in-world block is experiencing this problem, it seems to flicker as the player moves around it. At one point it also took on the texture of a nearby diamond chest from the Iron Chests mod.

Graves should be disabled by default, as they're soon being removed. I recommend disabling them.

D'oh. Oh well! Here's hoping you come up with a good alternative in the future--my players really liked them!

This isn't our bug, don't blame me :P
This is a known bug with either Forge or Optifine. All mods do it occasionally, it depends on a render order thing