The bug

Once a fire is placed down using a fire charge on top of this fire creates a brief visual bug where another fire is shown on top of it for a split second, this only works with fire charges and not a flint and steel. This also uses up a fire charge in survival mode

To reproduce

Simply create fire through natural or manmade means (this can also be soulfire) and then click on top of the fire with a fire charge and the visual bug should occur.