I assembled a minecart contraption for automatic harvesting of wood - mostly from vanilla and Create components but also included a Tinker's Bronze Barrel - picked it up with a wrench, and then immediately exited the world and closed the game (I'd finished playing for the day). Came back, reopened the game and world, and the contraption had disappeared from my inventory. It wasn't invisible, it just wasn't there.

Nevermind, it didn't disappear, I had just severely misplaced it. There's no bug here :)