When you go near the worldborder you are able to stack crystals and they can instantly crash a server with the right amount stacked. This is a bug within Minecraft as near the border there are some chunks which are "lazy".
MC-116336
Stacking Crystals at the worldborder
Resolved
Duplicate
0
Damian Benton
(Unassigned)
Unconfirmed
Minecraft 1.11.2
Retrieved
Caused by or duplicates MC-92901. The problem is that they are placed offset.