The goal line......except (and I know precisely where you are going with this) that is not what the Law says (although I am 100% certain that was the intent).
[LAWS]20.12 OFFSIDE AT THE SCRUM
(g) Offside for players not in the scrum. Players who are not in the scrum and who are not
the team’s scrum half, are offside if they remain in front of their offside line or overstep the
offside line which is a line parallel to the goal lines and 5 metres behind the hindmost player
of each team in a scrum.
Sanction: Penalty kick on the offside line
(h) If the hindmost foot of a team is on or behind that team’s goal line, the offside line for
scrum halves and non-participants is the goal line.[/LAWS]
A strict reading of the Law is that, if the hindmost player's foot is not behind the goal-line, then the defenders could have to defend up to 4.99 metres into their own in-goal.
Clearly, this is a mistake (the 12 year old proof readers strike again) but we all know what they mean.
This is what you get when you adapt an old Law without thinking about the dynamics of the situation. Under the old Law, the hindmost foot WAS the offside line. They simply added 5m to the old Law without accounting for the dynamics of doing so.
A simple amendment to (h) would fix it
(h) If the hindmost foot of a team is on or behind
or within 5m of that team’s goal line, the offside line for scrum halves and non-participants is the goal line
Although that brings up another point. What are the scrum halves doing in that clause? Scrumhalves have their own defined offside lines.. different for each scrumhalf.