Put the summary of the bug you're having here

What I expected to happen was...:
1st story. I have created a underground cave under my house, and had lit in the cave but enough to ensure that I thought that no mobs could spawn in. 

2nd story. I was coming up from the underground room from my house and my house is well lit, to go do something. At this time it is minecraft night time. 

What actually happened was...:
When I went to go get something on the ground level, I was shocked to my surprise to find an enderman in there, giving me the scare of my life. I killed it and lit up the area more , as I didn't want to occur again.  

2nd story. I come up from the stairs to find a zombie in the corner of my house. Yet surprising me yet again...

Steps to Reproduce:
1. Mine your own business playing the game
2. Make a room under the ground in a semi lit room but enough that you think that mobs won't spawn.
3. It will happen at any time when you don't realize...