A Wasp Infestation Local Lightning Strikes 4G Cellular Communications A Microwave Oven 
Answer: A Microwave Oven
For over a decade astronomers working with the Parkes Radio Telescope in Australia had been baffled by a mysterious interference in their observations that was difficult to pin down. The researchers took to calling the mysterious signals “Perytons” after the mythical bird-stag hybrid animal found in Jorge Luis Borges’ work Book of Imaginary Beings.
What was most puzzling (and perhaps a bit annoying) to the scientists was that the signal was obviously terrestrial in origin which meant it was of little use to them in terms of research and certainly not an indicator that a distant civilization was transmitting anything. Guesses about the origin of the signal were tossed about with the most prominent hypothesis that it was electrical energy in the atmosphere, that the blips that appeared in their data were caused by lightning strikes.
After seventeen years the mystery was finally solved when, in January of 2015, the facility was outfitted with new interference detection equipment. The source of the unidentified Perytons? The facility’s microwave oven. It turns out that although the microwave oven was properly shielded, if you opened the door while the heating was still under way (as opposed to waiting for the countdown timer to finish and the microwave beam to turn off), a brief burst of microwave radiation was released. While that amount of radiation is not even an issue in terms of safe operation of the microwave, when the radio telescope was oriented toward the facility housing the kitchen that little burst was easily detected as the telescope is designed to detect incredibly faint signals from light years away.
So why didn’t the scientists put two and two together sooner? After all, if someone came back from lunch and their associate was like “Johnson, you won’t believe it, we got another burst of those Perytons while you were away at lunch!”, you’d think eventually someone would go “At lunch you say? Hmmm…” The reason the mystery endured for so long, however, was because most of the researchers worked remotely and the vast majority of the time the microwave was used it was by local support staff who weren’t actively monitoring the telescope.
Image courtesy of the CSIRO Parkes Observatory.