Problem
The rune-covered sling "Whirlwind" is supposed to grant fire resistance, but this does not happen.

How to reproduce it
With a PC that doesn't have intrinsic fire resistance (or that does have it, but isn't wearing any other items that grant it), put Whirlwind in the missile weapon slot, then cast the Revelation spell, or drink a potion of insight. You'll notice that while Greater Identify says "It grants resistance to fire.", fire resistance isn't acquired or improved by equipping the sling.

Notes
I believe the Guidebook says the +Fire is only given when the sling is wielded - however, during testing, my PC's fire resistance wasn't enhanced by using it as a missile weapon, a missile itself, or putting it in either hand.