Sweden's air raid alarms usually blare our in unison at 3pm on the dot at the start of the first week in June, but they weren't heard in some areas of the capital on Monday.
Defence geeks turned to Twitter to voice their concerns and news of the missing safety test quickly became the top story on Swedish news websites based in Stockholm.
Charles Ågren, chief operating officer for Stockholm fire and rescue services, told The Local:
"We send out the alarm for very many municipalities, from several different computers. What happened in this instance is that we missed sending the alarm from one computer, so it did not go out to a number of municipalities in Stockholm county."
When asked if the error was a result of somebody forgetting to press the right button, he added:
"Yes, you could put it that way, although it's not a button as such. In principle, it was down to the human factor."
The test usually consists of a general alarm sounding out for two minutes, followed by a 90-second gap before another "all clear" signal.
It is a somewhat iconic sound that also has a symbolic role in suggesting a change in the season for many Swedes, due to its quarterly testing — although heavy rain in many places did not demonstrate that summer was underway on Monday.
Sweden regularly tests its alarm system as part of efforts to ensure that the nation would be prepared to alert people to a major emergency such as a fire, extreme weather or an attack.
The next demonstration is set to take place on September 1st 2015.
Anyone who hears the alarm on a non-test day is advised to remain indoors with the windows and doors closed, switch of any air conditioning systems and to listen to Sveriges Radio (Sweden's public broadcaster) for further information.