Sleepium

If you played with Sleepium before 1.1.4 together with C2ME (or another mod that saves chunks on its own threads): chunks saved during that time may be broken - parts of them missing, the log saying No key palette in MapLike[{}], terrain that looks hollow or does not appear. 1.1.4 saves them correctly from now on (checked by generating with C2ME, restarting and reading every chunk back: no errors), but it cannot bring back data that was never written. Regenerate those chunks: delete them with MCA Selector (or the region files of that area in saves/<world>/region), or start a new world. Anything built in those chunks is lost when they regenerate.

• World creation crashed with datapacks that make the world very tall (found testing JJThunder To The Max): ArrayIndexOutOfBoundsException in the aquifer generation once the world reaches past y 2047. Sleepium's aquifer shortcut read grid points whose height no longer fits the game's packed block positions; such cells now take the game's own path. The generated terrain is the same as without Sleepium (per-chunk hashes compared).
• Next to C2ME, chunk saving could fail and world generation stall (crash reports, "Failed to save chunk"): C2ME saves chunks on its own threads, and Sleepium's off-thread chunk saving shared its state between threads. Each thread now has its own; checked byte for byte against the game's own encoding (286 628 sections, 0 differences).
• With JJThunder To The Max on a 16-thread CPU (pre-generating with Chunky): vanilla 30 chunks/s, C2ME 48, C2ME + Sleepium 56, Sleepium alone 93.