In Pool D of the '99 world cup, 3 teams finished on 4 points - Wales, Samoa and Argentina. They were ranked by for/against that time, but trying to rank them by who won against the other is impossible, so maybe that's why it was done on differential on that occasion.
I'm back to having no idea
Found two from '87.
Pool C - Fiji, Italy and Argentina finished on 2 points, but again is impossible to rank by who won against the other, so went by differential.
Pool D - France and Scotland both on 5 points, they had a 20-20 draw, so went on differential.
Rankings at the completion of the pool phase
At the completion of the pool phase, the teams in a pool are ranked one through five based on their cumulative match points, and identified respectively as winner, runner-up, third, fourth and fifth.
If at the completion of the pool phase two or more teams are level on match points, then the following criteria shall be used in the following order until one of the Teams can be determined as the higher ranked:
The winner of the match in which the two tied teams have played each other shall be the higher ranked;
The team which has the best difference between points scored for and points scored against in all its pool matches shall be the higher ranked;
The team which has the best difference between tries scored for and tries scored against in all its pool matches shall be the higher ranked;
The team which has scored most points in all its pool matches shall be the higher ranked;
The team which has scored most tries in all its pool matches shall be the higher ranked;
Should the tie be unresolved at the conclusion of steps 1 through 5, the team that is higher ranked in the updated Official IRB World Rankings on October 3, 2011.
Rankings at the completion of the pool phase
At the completion of the pool phase, the teams in a pool are ranked one through five based on their cumulative match points, and identified respectively as winner, runner-up, third, fourth and fifth.
If at the completion of the pool phase two or more teams are level on match points, then the following criteria shall be used in the following order until one of the Teams can be determined as the higher ranked:
The winner of the match in which the two tied teams have played each other shall be the higher ranked;
The team which has the best difference between points scored for and points scored against in all its pool matches shall be the higher ranked;
The team which has the best difference between tries scored for and tries scored against in all its pool matches shall be the higher ranked;
The team which has scored most points in all its pool matches shall be the higher ranked;
The team which has scored most tries in all its pool matches shall be the higher ranked;
Should the tie be unresolved at the conclusion of steps 1 through 5, the team that is higher ranked in the updated Official IRB World Rankings on October 3, 2011.
Someone asked before what happens if two teams finish on the same number of points. If that happens it comes down to who won the match between them first, then for and against.
Currently pool B is:
England - 14
Argentina - 10
Scotland - 10
So assuming Argentina get five points against Georgia they'll go to 15 points, then if Scotland beat England by more than seven points England will get knocked out.
Yes it's unlikely, but we're already seems the likes of Ireland knock over Australia after Ireland struggled to get past the USA.
Scotland were the better side, looked more likely to score and dominated territory. Just couldn't cash in on the try in particular while Argentina pulled the rabbit out of the hat.