Both my husband and I play Minecraft, recently since server updated to 1.8.4 we have both fallen into the Void several times regardless of the Y coordinates. No where near bedrock some of the coordinates were Y=91 , Y=85 and another time at Y=25 all positive numbers it is usually preceded by lag. A couple of other players have also reported this on the same server. I don't know what their Y coordinates were. We have tried using previous versions 1.8.1 and 1.8 same effect. However Server is running 1.8.4 as were we when it started happening. Only way to try and survive is Control Alt Delete, but doesn't always work. Using 64bit Windows 7 Ultimate Edition. i7 3770k Intel Processor, 32Gig Ram, Radeon R9 290 Video card with 4Gig video ram.
Until recently this had NEVER happened to either of us.
It is not a modded or plugin server it is Vanilla.