I've spent the last hour trying to find the original quote, but it might have been a press conference. I distinctly remember an AFL official stating that should Buddy be escorted off the field for protection after his 100, then he would need to re-enter the field through the interchange gates in order to keep playing.

He was taken by security down into the rooms, and then 5 or so minutes later, led back onto the field without re-entering through the gate.

I mentioned this at the time, without knowing that Hawthorn would spoil our day and the one chance we're likely to see of two players kicking their ton on the same day in the same game.

Players leave the field all the time without usng the interchange bench. Every time they cross the boundary line chasing the footy or tackling they have left the field of play but they are allowed to return. This happens when a player is stretchered off or limps off with a serious injury and the coaches know he won't be back for the rest of the day.
It only becomes an issue if they are replaced by another player whilst they are off the field and if they had left the field outside the interchange race.
Franklin left the field and returned. Nobody replaced him so it was OK.

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Let slip the Blues of war (with apologies to William Shakespeare) (and Sir Francis Bacon, just in case)