I originally thought this issue was happening only when I added or removed items/mods, but it's happened in the last 48 hours, with no changes occurring to anything other than JAS mob spawn caps.

It doesn't seem to affect ALL blocks in an area, or even on the server... It may be restricted to certain chunks, based on what I've seen today.

I don't have images from the last couple of times it happened, but I do from this time:

Server .jar: Thermos | https://github.com/CyberdyneCC/Thermos | 1.7.10-R0.1-SNAPSHOT | 1.7.10-1614.58 (MC: 1.7.10)
Mod Version: v3.3.8_dev_r2 - MC 1.7.10.jar

I've seen reports of this happening and have experienced it once before. I was never able to track down the cause. It is likely something being shared between render calls -- do you happen to be using multi-core rendering?

I'm not sure I understand how a player using multi-core rendering can cause a server's blocks to change? Or am I misreading that?

I believe there are a few who DO use multi-core, including the girl who built this, and I've used it in the past but haven't in a few weeks.

There are known rendering issues when using Optifine, if multi-core rendering is enabled.

Can you explain how that would cause a server's blocks to change? I don't understand. This is more than a visual thing, which is what I consider rendering issues to be. The blocks have entirely different items in them.

I don't believe the block properties are changing, they're just rendering using incorrect data. Whether this is happening within this mod's code or in Forge/Optifine/Minecraft, is difficult to say.

Everyone who is on the server sees this, though, it's not just a visual thing. When the blocks are broken, they drop the items that they have changed to be holding (whether or not those items could be placed on carp blocks, to begin with).

Sorry if I'm being dense, just trying to wrap my head around this.