When you cast the fishing rod while looking straight down the server freezes. This can be seen for example by punching mobs, trying to retract the fishing rod, ... or in case of a multiplayer server the server will be shutdown by watchdog.
Using Fishing Rod Looking Straight Down Freezes the World
Bobber won't despawn when un-holding fishing rod when aimed straight down
Fishing rod make the game entities freeze
Fishing line dropped vertically down is falling very fast and glitches