Quantum Collision Issues

Despite having worked on and tested the game for months now, I ran into a rather severe bug today. Before I waste time with explanations, just take a look:

As you can probably imagine, the second barrel isn't supposed to just phase through the block like that. Seems like the issue is having two objects on the same block, right? Well...that's what I thought at first, but things got a lot weirder the more time I spent digging around. And I mean weird. Look at this stuff:

Now what is going on here? Once I read up on some quantum theory (as well as black magic for good measure) I'll let you know.