THE POPULAR saying goes that you can only learn by making mistakes.

So, what can be learned from the interchange steward's error that handed Carlton a goal during its narrow win over the Sydney Swans on Friday night?

One thing that can be gleaned from the situation is that, despite the introduction of the substitute rule, the interchange benches remain seriously hectic places.

And it's not just the officials who become confused by the swirl of activity as players come on and off the ground.

On Sunday evening at the MCG, Western Bulldogs midfielder Daniel Cross took three steps onto the field then ran back over the boundary line after fearing he had entered the fray too early.

Meanwhile, the Dogs' interchange marshal was furiously waving his arms as three other players tried to work out whether they should be coming off the ground or heading back onto it.

Despite their "Brown's cows" approach, the Bulldogs didn't break any rules.

As the AFL acknowledged on Monday, the Swans didn't do anything wrong either.

In fact, when the sideline official waved his red flag because he believed Dan Hannebery had become their 19th man on the park, they were actually penalising themselves by having five players on the bench.

There is no doubt the AFL official in question, whose mistake will result in him being stood down for a week, should have been paying closer attention.

But is it inevitable that errors will continue to be made in such a chaotic environment?

And if that is the case, has Friday night's "bench-gate" blunder demonstrated that the penalty for an interchange infringement - a free kick and a 50m penalty - is too harsh?

A bit of background first. The penalty was introduced after the Swans - yes, the Swans - had 19 men on the field for the final 35 seconds of their draw with North Melbourne at Etihad Stadium in 2008.

In the following week, they were fined $50,000, of which $25,000 was suspended.

The fine infuriated just about everyone connected with the Kangaroos, who believed the Swans should have been docked their two premiership points for the offence.

If such a penalty had been handed down, North Melbourne would have finished sixth on the ladder and hosted a final.

Instead they had to travel to Sydney in the first week of September, where they were beaten and eliminated from the premiership race.

Since then, there have been a few notable cases of players prematurely stepping onto the field.

Probably the most famous example happened when Hawthorn and St Kilda played out a draw at Etihad Stadium last year.

As was reported on afl.com.au, "the Hawks thought they had the game sewn up with three minutes to play in the final term when livewire forward Cyril Rioli put the side 13 points up.

"However, the emergency umpire had signalled an interchange infringement against Hawk Grant Birchall, Rioli's goal was cancelled and the Saints were awarded a free kick and a 50m penalty.

"What ensued was a mad scramble by both sides to add something to the scoreboard before a Nick Riewoldt behind followed by a stunning Ben McEvoy goal left the scores tied at the final siren."

After the game, Hawthorn coach Alastair Clarkson was far from happy.

“If it’s an infringement or not, there’s a blade of grass in it,” he said.

“To cop such a severe penalty and have the game changed with 50,000 [people] not knowing what’s going on and orange flags being waved... [stepping over the interchange line] has no bearing on the outcome.

“That rule has been introduced to stop blatant 19-men on the field kind of stuff and this is hardly blatant.”

Now human error on the part of the AFL official overseeing the benches has cast further doubts over the rule's validity.

“I suppose the one thing we’d say is that we’d think the interchange steward should err on the side of caution," Swans football manager Dean Moore said on Monday.

"If they’re going to indicate there’s been an infringement and a penalty awarded, I reckon you have to be absolutely 100 per cent sure that is the case.

“It’s such a big penalty, particularly in a tight game.”

So, where to from here?

Given there has never been an allegation levelled at a player that he deliberately tried to enter the field early, ditching the 50m penalty and just paying a free kick might be a good way to make the punishment better fit the crime.