Windows 10, nVidia GTX970
Short description of error
When file is saved in 2.79 and opened in 2.78, compositing tree is broken in some nodes. For example in 2.79, Render Layer outputs are Depth and Speed, while in previous versions is Z and Vector. When that file is opened in 2.78, anything connected to those outputs is disconnected. Hue Saturation Value node, when opened in 2.78 becomes red and titled "Undefined"
Exact steps for others to reproduce the error
Open Blender default scene in 2.79. Enable vector pass in render layers/passes. In compositing nodes, add vector blur node, and connect Depth output to Z input and Speed output to Vector input. Add a Hue, Saturation Value node after it, and connect it to Composite. Save the scene. There are probably other nodes that don't work.
Open the scene in 2.78. You'll find that Vector Blur node has been disconnected from Render Layer in it's Z and Vector inputs, and Hue Saturation Value is now red and "Undefined"
This file can be used as a test file to quickly see what's happening: