The bug
When in a boat with a mob (tested with dog and cat) riding in back, shooting arrow causes damage to the mob riding in the boat.
The arrow appears to start slightly behind the player (can be seen when viewed in F5) so it hits the mob's hitbox first. The mob's hitbox appears to slightly overlap the player's.
How to reproduce
Videos:
https://youtu.be/eXIonOnBD_M?t=16m40s (Before 16w34a)
https://www.youtube.com/watch?v=J9jEQjGz0Vs (16w34a and above)
Linked issues
is duplicated by 3
MC-99676
Player unintentionally fires arrow at non-hostile mob while both riding in boat
Resolved
MC-154485
Arrows fired from bows & crossbows hit your passenger in a boat
Resolved
MC-184358
Arrows fired from a boat, while another passenger is aboard, hits the passenger when not shot at them
Resolved
Something about this has changed with the 16w34a snapshot. The angle of the bow when fired seems to be key. I have some footage showing it being replicated in this snapshot, albeit less frequently here: https://youtu.be/UZRPlWUfdnk