So far after testing hexxit on a server with multiple players, we've noticed some bugs. Infernal Mobs, Battle Gear, Primitive Mobs, and Chocolate Quest. Those four tend to be the most common mods to crash the server or client. We've been unable to detect the source at the time, but we can say this:
Infernal Mobs: Gets picky when some non-vanilla mobs become 'boss-mobs' (or Infernal) and have intense abilities and effects. With Primitive Mobs, it tends to really mess up with Brain Slimes most commonly. Chocolate Quest mobs as well! So just be careful.
Primitive Mobs: Crazy buggy with Infernal Mobs, as explained above. Even more so with Infernal Mobs and Random Things during a Blood Moon.
Mine & Blade Battle Gear 2: Difficulties and crashes with Skeletons, custom arrows, and weapons all within the mod itself. Weather it's alongside the mods mentioned above, or it's not stable yet is beyond us. However duel wielding non-BattleGear tools/weapons returns no errors or crashes. Strange, right?
Chocolate Quest: Same problem as before and even in original Hexxit (1.5.) Just world generation, and spawning the structures. With Infernal Mobs, things get bad fast. Sometimes the mods don't play nice and can result in a big crash both server and client. However this is normal as this has always been the norm. A quick restart of both server and client, and everything is fine. The world is also fine as we've tested. So in the end not too bad, but can be a problem if you're out and about and suddenly something from this mod spawns in your area and crashes your client.
TLDR: Be careful with the mods listed above as they've caused the most common problems.
ps,
We hope to further finish testing to release the first official stable release of Hexxit REvisited, after this current rolling-update! (yay!)
