Reproduces in Mariculture 1.2.1e. Pastebin of crashlog: http://pastebin.com/hQuY5sBr

Steps to reproduce:

1. Create a Mirror. Craft it or cheat it in, doesn't matter.
2. Enchant the Mirror with Unbreaking I. It doesn't matter how, you can use it to enchant itself or use an Enchanting Table.
3. Open the Mirror's GUI and attempt to place anything in the enchantment slot (tested with pearl ring, snorkel, pickaxe). The client immediately crashes.

I probably won't fix this until 1.2.2, i'm too far in to go back and release an update. This avoidable for now. Pretty sure it will only happen with the basic mirror. This is because I'm checking if the item has extra nbt data then calling the mirror levels, that's set when you set the magic/celestial mirrors. The normal mirror doesn't have these levels not a nbt tag, but when you enchant it does have nbt, so when i tries to read the levels from the mirror, there's no levels to read because it's the basic one, and therefore crashes....

Fixed in 1.2.2