For the soldiers guarding the Habad Loubavitch synagogue in Vincennes in southeastern Paris it came as "a surprise", was the deadpan verdict of the police source.
The incident happened on Thursday evening when members of the synagogue were celebrating the festival of Purim, during which followers often dress up in costumes and give each other presents.
The source said the man arrived wearing long North African robes, carrying a fake Kalashnikov and crying "Allahu akbar", Arabic for "God is greatest".
"It created a certain emotion among the soldiers," the source said.
The initial shock passed quickly when members of the synagogue realized they knew the man, and that it was only a joke.
He was nonetheless ordered to present himself at his local police station on Friday.
Soldiers have been posted outside many Jewish buildings and other sensitive locations around France in the wake of deadly jihadist attacks last year.
Purim celebrates a story from Hebrew scripture in which the Jews of the Persian empire were rescued from annihilation. Every year, Jews celebrate by giving each other presents and wearing costumes.