Version
Minecraft 1.12.2
Silent's Gems 2.6.5
Silent Lib 2.2.16

Right so I'm using silent's gems and i found out these nifty Holding Gems. I use it for stone, no big deal...

Except i checked the usage of this item. Apparently it can have mending on it? I was curious, so i threw mending on it...

Mending didn't work, so i paid it no mind...then i took it to a flux reconstructor, a machine from another mod that repairs items using experience... I didn't think it would work, but...

I put it in for a few second and uh...

I think this is a very critical, if not gamebreaking bug. This only happens on the holding gems, normal chaos gems and whatnot are not affected.

Somehow I'm not surprised. Thanks for reporting.

Good guess on the Mending by the way, that was reported and fixed not too long ago (#233).