Coin duplication
Coin duplication is a glitch that can cause the rock blocks in the Cascade Kingdom that normally drop a ring of eight coins to drop duplicate coin rings, in order to harvest up to 48 coins. The glitch involves hitting the blocks at a precise angle while using the T-Rex capture, and can be performed in the Cascade Kingdom overworld as well as the Dinosaur Nest room. When used in the Dinosaur Nest, it is the fastest known way of accumulating coins excluding Luigi's Balloon World.
Coin duplication is only possible on versions 1.0.1 and later, since it utilizes extra hitboxes of the T-Rex that were first implemented in 1.0.1.