When I create/login into a newly generated world in single player, mobs don't have responses, don't take damage, and blocks, (survival mode) break
a few seconds after I actually broke them. Once I login again, after a minute aprox, I start to experience lag, that is like being in slow motion, with occasional lapses of freezing lag. If I go to survival, I don't experience lag,
but actually being teleported to Y:30 into stone. I don't suffocate, but end up
having to dig out. In Creative, lava doesn't flow, and there are no chunk
updates. (Just to let you know, It could just be my computer, but still reporting just in case.) You can't really activate this bug, I have found out,
it just occurs randomly at the start of most worlds I create. (Say, 85 percent of all). Strangely enough, this bug doesn't occur to me in Multiplayer.

PS, sometimes, (Quite rarely though, if I break/place blocks, logout, and back in, the blocks I have placed/broke disappear and the world is
"reset" although if I place blocks on grass, and the bug occurs, I find the grass as dirt.