The best thing to do if you get an email that's obviously an accident? Stay the heck away from the "Reply All" option. Employees of the UK's National Health Service learned this the hard way after a "test" email was mistakenly sent to 840,000 of them, the BBC reports.
According to a Tweet from NHS employee Olly Benson, the email got at least 390 "reply all" replies, requesting to be unsubscribed. Although that's over just 0.04 percent of the number of employees, those replies each went out to 840,000 people. That's at least 327.6 million emails. Although the list has been disabled, according to an NHS spokesperson speaking to the BBC, Benson reports the replies are still rolling in.
As a result, the NHS' email servers were running very slowly. The NHS Pensions Twitter account warned people trying to contact it via email might experience delayed responses.
The NHS did not immediately reply to a request for comment. The request was sent via email, so it may be some time.