From BlenderWiki

Blender 2.79 Release Notes

Work in Progress

Not all new features or changes have been added here.

Important forward-compatibility warning: Any .blend file using new ID Pointer custom properties saved in Blender 2.79 and beyond will crash when opened with Blender 2.78c and any previous releases (follow link above for more details). This does not affect any existing custom property type, only a new type introduced in 2.79.

Testbuild available!

https://download.blender.org/release/Blender2.79/