Grounding flights of fancy.
A mysterious warning from a Belgian cell phone shuts down a shopping district in Amsterdam for a whole day.
A Yemeni's gifts for relatives arouse suspicion and land him - and another Yemeni he's never met - in a Dutch jail for two days.
These and many other recent episodes proved to be false alarms, yet paralysed life for hundreds, perhaps thousands, of people. More than nine years after 9/11, intelligence agencies, police and airlines are still trying to balance protecting society from possible terrorism without derailing routines. "It's disturbing the way these kinds of situations can disrupt day-to-day lives," said Bibi Van Ginkel, an officer based at the International Centre on Counter-Terrorism in The Hague.
Authorities are unlikely to let down their guard any time soon, she said, because "they know that these kinds of little tips could easily be a real threat".
But she added that protocols are constantly being reviewed to weed out cranks and hoaxers.
The latest false alarm played out on Saturday in Sweden when Stockholm police evacuated a Pakistan International Airlines (PIA) jet and a SWAT team arrested a Canadian man after an anonymous caller in Canada tipped-off authorities a suspect was carrying explosives. No explosives were found, but passengers waited nine hours before continuing their flight.
The suspect was released on Saturday night. Royal Canadian Mounted Police spokesman Sergeant Marc LaPorte said police are investigating whether the call was a "terrorism hoax".
Swedish police defended their actions. Spokesman Kjell Lindgren said the Swedes acted according to guidelines set out for airline security when they let the plane land in Stockholm.
"With the information that we had, and when the captain doesn't feel he can guarantee the security onboard, I think we did exactly what should be done and it was handled according to the plans that we have," he said.
The difficulty for authorities lies in the fact that threats range from the ridiculous to the terrifyingly real.
In February, Irish Republican Army dissidents and a cab driver called in warnings to Irish police that a hijacked taxi carrying a bomb had been parked outside a Londonderry police station.
Nobody was injured in the blast though the explosives detonated some 20 minutes earlier than the bombers said it would. On the other extreme, a German traveller running late two years ago for his flight in Verona, Italy, phoned in a bomb threat to delay its departure.
Authorities closed the airport before tracing the call to the man.
Contrary to widespread belief, security at airports is not being beefed up across the board, and in some cases measures are actually being scaled back.
The European Union says it plans to phase out restrictions on passengers carrying liquids onto planes by 2013.
But police seem to have an unstated rule - When in doubt, act.
Van Ginkel said 100 per cent security is theoretically possible, but added it would be a bitter pill to swallow. "It is possible, but it is not going to be a state we like," Van Ginkel said.
"North Korea might qualify as having close to 100 per cent security, but luckily we don't live there."
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