If you have two fishing rods on your hotbar, cast the first one, then switch to the second one, it does not retract the fishing line. This allows for users to, for example, cast using their Lure III rod but switch to an unenchanted rod once they get the fish particles or actually hook a fish, and therefore take no durability on the Lure III rod.

This bug is similar to the old Minecraft bug with pickaxes, where you could switch between different pickaxes for optimum mining conditions, while only taking durability on one pickaxe.