I was sieging a player, JoeSchmoo21405, when a strange glitch happened that sent me straight through one of the stone bricks.  I dig a little area outside of his claim and built a dirt piller to get on top of his small stone brick box, when I jumped from the dirt pillar onto the box I went straight through the roof of the box and appeared inside it.  I was trapped inside due to the siege, so I killed him so I could teleport out.  I apologized and took nothing from the unlocked chests that were inside the base, due to me not knowing what happened, or how I got inside.  I returned the items he had on him when he as inside, which was a wooden pick, and some cobblestone.  I feel like this could have been some odd glitch with lag or whatnot but I still have no idea.