EVGA GeForce GTX 770 Classified 4GB GPU
2x Intel Xeon E5-2670 8-cre 2.6 Ghz
8 GB ADATA RAM @1833Mhz
Broken: Blender 2.79a (Hash: 5bd8ac9) and Blender 2.79.1 (Hash: 2ae6a93)
Short description of error
In short, I don't know for sure if it's a bug, or an intended limitation of Cycles nodes. I have over like 450 nodes, and the bump no longer works when certain ones are plugged into it, and instead acts as if it's a black emission node, as does the Principled BSDF if the Bump node is connected.
It's worth noting I had a friend test on his rig, and his own blender version, and it didn't work for him either.
Exact steps for others to reproduce the error
Just download the .blend, and it's already done for you. To "fix" it, simply delete the Reroute marked "REMOVE ME" or the Reroutes labelled "OR REMOVE US".