There are many bugs that are similar to this but they are not the same(eg. https://bugs.mojang.com/browse/MC-44023, https://bugs.mojang.com/browse/MC-42373).
I will do some further testing with my old unmodified Survival worlds and other stuff and report any new info.
I keep having the same issue. Game lagging for no reason, block reappearing and mobs freezing. But it's only happening in a map generated in 1.6 loaded on 1.7.
Ohhh. Thank you so much. I would ragequit if I had to do it again.
Anyway this doesn't mean the problem is fixed even http://www.minecraftforum.net/topic/1395030-allocate-more-memory-for-minecraft/ doesn't fix it. I crashed many times before and nothing like this happened...
The ram problem is also caused by bugs in the game because normally when i play even on max render and lot of entities around it takes like 60% of the space and sometimes it takes 100% even on peaceful and render distance 2 chunks...
I think I know where is the problem now. I'm so dumb. Please delete this report (I can't find a way to do it myself 😃).