Why do random blocks disappear on minecraft?

I'm trying to build a city, and whenever I add a bunch of building blocks, others I've already built keep disappearing! Why is this?

Guest

There's 2 reasons. It could be a crepper blowing up your blocks, or, an enderman taking your blocks.
If that's not it, maybe you're playing with someone whose trolling you.

Justin

Could be an ender man. But if there's a bunch of building blocks then it can't really. But your game could be running out of space and removes the blocks or maybe even it lagged and some blocks disappeared.

Jarod