If you set block on top of any two block mob. It's hurt the entity.
Environment
OS : Windows XP (x86) 5.1 Service Pack 3
CPU : Intel(R) Pentium(R) 4 CPU 2.80GHz
RAM : 1 GB DDR RAM
Java : Java Development Kit 1.7.0 Update 65 (Used for making my mods)
OpenGL : Intel 845G GL Version 1.3.0 - Build 4.14.10.4342
It's a solid block. It can happen with any solid block.