The cause of the 12 April gamma-ray flare, described at the Third Fermi Symposium in Rome, is a total mystery.
It seems to have come from a small area of the famous nebula, which is the wreckage from an exploded star.
The object has long been considered a steady source of light, but the Fermi telescope hints at greater activity.
The gamma-ray emission lasted for some six days, hitting levels 30 times higher than normal and varying at times from hour to hour.
While the sky abounds with light across all parts of the spectrum, Nasa's Fermi space observatory is designed to measure only the most energetic light: gamma rays.
These emanate from the Universe's most extreme environments and violent processes.
The Crab Nebula is composed mainly of the remnant of a supernova, which was seen on Earth to rip itself apart in the year 1054.
At the heart of the brilliantly coloured gas cloud we can see in visible light, there is a pulsar - a rapidly spinning neutron star that emits radio waves which sweep past the Earth 30 times per second. But so far none of the nebula's known components can explain the signal Fermi sees, said Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology, US.
"The origin of these high-energy gamma rays has to be some other source," he told BBC News. (read more)