I started up a new hardcore world in 21w44a, and very early on (first day still) I was chopping down trees when suddenly my game froze with a message in the lower right corner of the screen "Saving world". My CPU fans spun up to 100%. It just hung with an occasional spinning wait mouse cursor, and then eventually it offered up a screen telling me that the Java VM had run out of memory and that I could try returning to the title screen or quit the game.

I returned to the title screen and attempted to load the same world again and it just started doing the same thing and also spinning the fans up to 100% again.

I then restarted the game entirely and attempted to load the same world again and the exact same thing happened. I thought you guys might want to look at the world, so decided to hop on here and file a report. A casual search didn't seem to locate anything recent.

I started up a second world with the exact same results, except the game just crashed with no message eventually.

Update: I was able to get back into my existing worlds after modifying the Java arguments to specify more memory (I changed the -Xmx2G to -Xmx16G).