Just after the Super Secret Event ended on 31st March 2016, the Scatterbugs unleashed their rage and attacked Pokémon Vortex in an attempt to take over the Grass Maps.
For five hours, instantly after the event ended which required you to catch Scatterbugs and evolve them into Vivillon (Fancy) or Vivillon (Pokeball), all that could be found on the grass maps during the day were it's usual legendary Pokémon and Scatterbug.
The top of the maps displayed the following error: System Overload. Error Code: #664 - Could not parse Scatterbug.
A hint in the error code to point at the fact it was an April Fool's joke is that #664 is the national Pokédex number of Scatterbug itself.
An hour after this occurred, Pokémon Vortex updated it's Facebook and Twitter status' to explain that the maps had been overrun with bugs.