I began staring at an Enderman and it wasn't becoming hostile towards me. I assumed I was not watching it directly in the face properly. I then proceeded to attack the Enderman and it became hostile but for some reason it was not able to inflict damage on me. The Enderman teleported and when I found him again I began staring into his face again but it was still not becoming hostile. The game was not on peaceful mode.

Proof I was not in peaceful mode and seed are in the pictures below.