All about a door

This was actually the same campaign as the last story about the magic weapon. The party mage had received early on a Robe of Useful Items. basically its a robe with a bunch of patches that you pull off and and they become various medium to large useful items like ladders, rope, hammers, that sort of thing. 

Robe of Useful Items: This appears to be an unremarkable item of apparel, but a
wizard who dons it will note that it is adorned with small cloth patches of various shapes.
Only the wearer of the robe can see, recognize, and detach these patches. One patch can
be detached each round. Detaching a patch causes it to become an actual item, as
indicated below. A robe of useful items always begins with two each of the following
patches:
dagger, lantern (filled and lit), mirror (large),pole (10-foot length), rope (50-foot coil)
sack (large)
In addition, the robe will have 4d4 items which must be diced for. Roll 4d4 to
determine how many additional items a robe has and then percentile dice to determine
specific items:
D100 Roll Result
...
09-15 Coffer (_' x _' x 1'), silver (500 gp value)
16-22 Door, iron (up to 10 feet wide and 10 feet high)
23-30 Gems, 10 of 100 gp value each
...
Multiple items of the same kind are permissible. Once removed, items are never
replaced.

The mage carried this robe around all summer int he game and in the climactic battle with a lava dragon in the base of a volcano he cast fly on himself. flew up 300' and pulled off the patch for the iron door and dropped it on the dragons head. Now there was much discussion about how to rule about this finally we decided that the dragon would take something like 30d6 damage if the door managed to hit him, the mage then proceed to roll a natural 20 on the attack roll which in my game means a guaranteed hit and maximum damage. The dragon ended up taking 2/3rds of its hitpoints in damage.