The bug
Nether portal blocks do not generate correctly. That's really all about it.
How to reproduce
Build a nether portal
Enter the nether portal
Turn around and see the glitched nether portal blocks
Environment
Microsoft Windows [Version 10.0.15063]
Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz, 3.40 GHz
64-bit operating system, x64-based processor; 8.00 GB RAM
NVIDIA GeForce GT 630; GeForce Game Ready Driver Version 385.28
Java 1.8.0_144-b01 (64-bit)
Yup, I got the same thing too!