If one player is hosting a LAN multiplayer server, another player joins, and one player observes the other player fishing from several chunks away, the black pixel line is perceived as insanely thick.

To reproduce (if it wasn't obvious how to do so):
1. Host a server. (I experienced this on LAN.)
2. Have other players join.
3. Have a player fish, while another player positions themselves at a fairly long (2-4 chunk?) distance from the player that fishes and observes it.
4. Notice how thick the line seems to be. It looks worse when the renderer distance is relatively small.