Turning off Bluetooth and Wi-Fi when you're not using them on your smartphone has long been standard, common sense, advice. Unfortunately, with the iPhone's new operating system iOS 11, turning them off is not as easy as it used to be.

Now, when you toggle Bluetooth and Wi-Fi off from the iPhone's Control Center—the somewhat confusing menu that appears when you swipe up from the bottom of the phone—it actually doesn't completely turn them off. While that might sound like a bug, that's actually what Apple intended in the new operating system. But security researchers warn that users might not realize this and, as a consequence, could leave Bluetooth and Wi-Fi on without noticing.

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"It is stupid," Collin Mulliner, a security researcher who's studied Bluetooth for years, told Motherboard in a Twitter chat. "It is not clear for the user."

To be clear, and to be fair, this behavior is exactly what Apple wants. In its own documentation, the company says that "in iOS 11 and later, when you toggle the Wi-Fi or Bluetooth buttons in Control Center, your device will immediately disconnect from Wi-Fi and Bluetooth accessories. Both Wi-Fi and Bluetooth will continue to be available." That is because Apple wants the iPhone to be able to continue using AirDrop, AirPlay, Apple Pencil, Apple Watch, Location Services, and other features, according to the documentation.

Motherboard tested this behavior on an iPhone with iOS 11 installed and verified that Bluetooth and Wi-Fi remain on in the settings after turning them off in the Control Center, as some users have started to notice.

Andrea Barisani, a security researcher and one of a few people to notice this change, said in a Twitter direct message that the new user interface is not obvious at all and makes the user experience more "uncomfortable."

Turning off Bluetooth and Wi-Fi reduces your exposure to potential attacks to hardware, firmware and software, so "it's good practice," Barisani told me. Just last week, security researchers revealed the existence of a series of bugs in the way some operating systems implemented Bluetooth that allowed hackers to take over victim's devices as long as the Bluetooth was on—without needing to trick the user into clicking a malicious link or do anything at all.

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It's worth mentioning that both Bluetooth and Wi-Fi will become active again when you toggle them off in the Control Center at 5 AM local time, according to Apple's documentation. It's unclear why that is, but just so you know.

Apple did not immediately respond to a request for comment.

The takeaway is that if you want to really and completely turn off Bluetooth and Wi-Fi on iOS11 you can't do it from the Control Center anymore, you'll have to do it through the Settings app.

This story has been updated to point out that Barisani was only one of a few others who noticed the change.