I used a pulley to bring andesite from bedrock up to the surfaces and every once in a while... the machine vanishes into bedrock.
I've included an image of the end result.

And here is an image of how it is usually set up.

Some notes:
This is on a server that I run locally.
I always use /save-all and /stop to shut down the server
This happens after restarting the server and only some of the time.

Possibly related to Issue 84 on the Create issues page?

This issue can be reproduced (albiet inconsistently) when the pulley's chunk is unloaded too, not exclusively on server restart.

+1 been seeing this issue as well. I haven't done any extensive testing but as far as I can tell it seems to be happening when I move out of render distance of the chunk it's in, despite me force-loading the chunk through the FTB Chunks mod. The solution I'm going to try is replacing the pulley system with a gantry, since I have a gantry-based contraption in the same chunk that has never had this issue, but I am a little sad about it because I love the aesthetic of the pulley-based system. If this gets fixed I'll definitely be going back to the pulley