"After some extra testing, the server crashes only when right clicking on a Weeping Angel. Shows its only affecting the right click, pickaxe kills are fine but right clicking them to damage them seems to bug out the angels in many ways. Left click actions appear to function correctly. Eg. using a packaxe can kill them per usual without causing the server to crash."
http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1292523-weeping-angels-mod?comment=350

https://github.com/TheTemportalist/WeepingAngels/blob/master/src/main/scala/com/temportalist/weepingangels/common/entity/EntityAngel.scala#L542