If you summon the TARDIS inside itself then it causes a loop. That's not the exploit. If you break the TARDIS, then exit through the doors you will end up in the last place the TARDIS was, which was inside the TARDIS. The TARDIS will also generate again. The "Drop Tardis Item" config setting is set to TRUE by default, so if you break the TARDIS, go through the doors and break it again, you will now have 2 TARDISes in your inventory. This will go on until you use the TARDIS computer to materialise in another dimension. That is an incredibly powerful exploit that can have disastrous consequences on public servers. Please try to figure out how to prevent this.