By Andrew Fraser
BBC Sport in Athens
It took a man wearing Greek knickerbockers, a tutu and clown shoes to persuade Olympic organisers that their security precautions were not as good as they should have been.
The prankster's bizarre appearance during the men's 3m synchro diving competition has prompted the introduction of plain-clothes police officers in and around Games venues.
It followed claims in a British newspaper that an undercover reporter had been able to plant suspicious packages in the main stadium ahead of the opening ceremony.
He allegedly used names like Michael Mouse and Robert bin Laden on his pass.
On the face of it, both incidents are major embarrassments for a country which has spent almost £1bn on security.
But the Greek Public Order Ministry is adamant that all venues are safe.
For the Olympic visitor, security around the venues is certainly tight.
The main Olympic complex operates on a ring of steel basis, with a mixture of police and volunteers manning each entry point.
Spectators must pass through airport-style x-ray machines to get inside. Bags are screened and the contents are sometimes checked.
The same rules apply to the media.
Former Olympic 100m champion Donovan Bailey was not spared a search when he set off the metal detector on his way through the checkpoint to get into the International Broadcast Centre on Tuesday.
Inside the centre, a soldier brandishing an automatic rifle stands guard by the entrance to the headquarters of the host broadcaster.
He is just one of 70,000 security personnel on duty for the duration of the Games, and there is a sophisticated network of cameras to assist them.
Security was conspicuous for the three day event
It comes as no surprise that petty crime fell by 40% in July, given the increased police presence throughout the city centre.
Visitors have become accustomed to the whirr of helicopters in the sky, and every now and then you catch a glimpse of Phoebus the security blimp, which can detect chemicals in the air.
At the canoe and kayak slalom venue, two batteries of patriot missiles point into the sky, ready for any aerial threat. Nato forces are also on stand-by.
Security could certainly be tighter inside the venues, however - as the diving incident showed.
On the eve of the opening ceremony, printed signs by entry points to the stadium warned that photograpy was not permitted. But there was no one there to enforce the rule.
And it was possible to wander unchallenged into the stadium on day one of the Games as the clear-up operation began after the ceremony.
Athens organising chiefs will not be too concerned that their worst security threat so far has been a publicity-seeking Canadian in fancy dress.
But they will be only too aware that if they leave any loopholes in their defences for the remainer of the Games, the consequences could be far more serious.