On Wednesday, a computer glitch caused trading to stop at the New York Stock Exchange for more than half of the time markets were supposed to be open. They say that new technologies break in new ways, and that was the case yesterday, but they are also not immune to breaking in old ways: Twice, U.S. exchanges have been halted by … squirrels.

In 1987, a squirrel—through methods that remain mysterious nearly 30 years later—damaged the electrical system at a complex in Trumbull, Connecticut, where many of NASDAQ's computers were at the time. This led to an 82-minute shutdown that affected exchanges around the country and is estimated to have forced NASDAQ to trade at about 85-percent capacity.

That squirrel died, but its legacy lived on: In 1994, another mischievous Trumbull squirrel caused a similar disruption after gnawing on a power line. This development, along with other shutdowns around that time, left the chairman of the House subcommittee that manages securities trading "deeply concerned" about NASDAQ's overall stability.

As far as financial markets are concerned, these are freak occurrences, but squirrels commonly wreak havoc on America's wiring systems, as Alexis Madrigal explained in The Atlantic in 2011. By August of that year, the rodents were responsible for 17 percent of the damage done to the fiber-optic network of Level 3 Communications, a company with 84,000 miles of cable. That was down from 28 percent the previous year. "Honestly, I don't understand what the big attraction is or why they feel compelled to gnaw through cables," one of its vice presidents wrote at the time.

Squirrels are also responsible for many power outages across the country, and Jon Mooallem suggested in The New York Times in 2013 that they're frequent power-line chewers because their incisors never stop growing, so they constantly need to wear them down by teething. Also, squirrels are naturally drawn to transformers, whose nooks can be similar to the hollow spaces in trees.

In 1987, after the first Trumbull squirrel had done its deed, traders were forced to use telephones, a backup plan that one executive called "somewhat uncomfortable." But that's arguably a step up from when a computer failure struck at the NYSE in the late 1960s: For more than three hours, traders had to write their orders by hand and send them off in pneumatic tubes. Luckily, there was a squad of 24 women stationed at paper-tape punch machines, trained for just this sort of emergency, waiting on the upper floors to receive the orders and broadcast them to tickers throughout the nation.