Mobs can lose AI and become invisible

Today on my survival world, I was converting one of my old mines into an extension for my house. I had not yet found coal for torches when I was digging that mine, so it was dark and there were lots of mobs in there. When I unblocked the entrance, there were two creepers in the shaft. And they weren't charging me, they were just standing still. They didn't even explode when I walked past them. After I had lit the place up, they regained their AI and blew me up. When I returned to get my stuff, I kept getting attacks from nowhere. When I was killed by the attacks, the chat said I had been slain by a zombie. This happened two more times until the sun rose and killed the invisible zombie.