So, this is the biggest technicality in regard to Price Promise I've ever seen.
Bunnings has their OnGas Gas Bottle 9kg $0.01c cheaper than Mitre 10. Price Guarantee of 15% brings it down to $34.91 at Mitre 10.
*Currently out of stock in Rotorua. Check your local store before going to your local Mitre 10.
Mitre 10 link
Cool if so,
This might be one where they know they are priced higher and have updated their instore price already to match bunnings, so they their price guarantee wouldnt apply. Sneaky trick.