Anytime, even on a blockless world. I have never had such severe memory issues on my older, non-gaming computers. The launcher won't let me allocate more than xms1G or xmx1024M. On older versions I ran fine with **512mb** memory.

How to reproduce:
Go to new launcher, do the xmx1024M or whatever it is
Do a world that has a crazy creation*, (like many of mine)
Crashes to memory related things
(*This happens for me even on blockless worlds and multiplayer)

Sometimes this is so bad, my game closes without warning.