A goal is scored if an in-bounds player catches a legal pass and all of their first simultaneous points of contact after catching the disc are entirely within their attacking end zone (note 12.1, 12.2).
'Callahan' goal: The offence throws the disc, but a defender intercepts the pass in the Offence’s defending end zone (which is the Defence’s attacking end zone).
The defence scores a goal.
Rule 14.1 states that the goal is scored when a player catches a legal pass and the first contact with the ground is inside their attacking endzone. It does not state that the pass has to be from a teammate.
The term Callahan Goal is an unofficial term for this type of goal.
A player catches the disc in the attacking end zone, but does not know this and throws the disc.
It is a goal, no matter the result of the additional pass.
Rule 14.1 does not say that the player scoring the goal must be aware of it. So if somebody has a clear perspective on it and declares it a goal, it is a goal. The disc cannot be thrown away after a goal is scored, so that action can be disregarded.
If it is unclear if the player scored (ie there is no agreement on the player who had best perspective, and there are opposing view points on the play) the result of the additional play stands.
Players may ask for perspective from people on the side-line to determine if the pass was caught in the endzone, however it is still up to the players involved to make the final call.
If a player believes a goal has been scored they may call “goal” and play stops. After a contested or retracted goal call play must restart with a check and the call is deemed to have been made when the pass was caught.
After a contested or retracted goal call where the receiver maintains possession, all players should return to where they were when the pass was caught.
If a player could reasonably assume that they have scored a goal, and behaves as if they have scored a goal (for example, by celebrating a goal) this should be treated as a “goal” call. This “goal” call is a stoppage of play and the result of any additional play does not stand.