Happened when I was experimenting with a small gearbox setup. I would take a picture, but the game crashed before I was able to. I had one creative source powering one gearbox, I led an axle out, and then I placed another frame next to the axle, and when I placed a gear to get the second gearbox moving the game crashed. Crashlog:

https://paste.dimdev.org/fevajafira.mccrash

Reloaded the world, tried to figure out what happened by tinkering with it, and then wouldn't you know it, it crashed again. As such, here's another crashlog, so you can have two for the price of one:

https://paste.dimdev.org/masupovuqi.mccrash

Also, congratulations on your first bug report. Hopefully it will (not) be the first of many.

Mystical Mechanics: 0.1
Forge: 14.23.4.2749
MC: 1.12.2