How to reproduce:

1. Set the time to night and place a bed
2. Put a fishing rod in your off-hand and reel it out (It doesn't have to be in water)
3. Press F to switch the rod to your other hand and reel it in
4. Switch the fishing rod back in your off-hand and reel it out
5. Sleep in the bed. The line will appear in your right hand rather than your off-hand.