Q. My phone started opening apps and switching among them all by itself. What just happened?

A. When you see a smartphone or tablet go berserk like this, bouncing from app to app faster than you could hope to accomplish, it’s easy to think some sort of malware is at fault.

But the problem is this case probably lies in the device’s hardware — specifically, the digitizer under the screen that registers the taps and swipes of your fingers. If that layer of circuitry starts to go bad, spooky if not outright alarming behavior can result.

It’s a common result of water or moisture getting into a display, said iFixit founder Kyle Wiens in an e-mail. He added that a digitizer can start to go bad if a phone isn’t put together correctly, either at the factory or after repair.

I saw this happen on my own phone (I verified that this Android device hadn’t been hacked by doing a complete reset), and I can attest firsthand that a failing digitizer can make for a completely unusable phone.

No touchscreen gadget seems immune to this malfunction. A little searching quickly located reports of these touchscreen-freakout symptoms on an iPhone and an iPad; Android phones from Motorola, HTC and Sony; and a Nokia Windows Phone.

You may get some advance warning of this condition in the form of certain parts of your screen no longer responding to a tap. In Android, you can also check its accuracy with apps like the free Screen Touch Test. Or, if you’ve enabled the Settings app’s hidden Developer Options menu, you can activate a “Pointer Location” option there that draws a thin line everywhere the screen senses your touch.

If, however, your first sign of trouble is the phone going haywire, shut it down before the hyperactive digitizer can start calling or texting friends on your behalf. The problem may go away, in which case you should immediately do as complete of a backup as is possible.

Should this issue resurface, you might as well try a factory reset; here’s Apple’s advice and here’s Google’s on that procedure.

But if that, too, fails, you will probably have to replace the digitizer — which usually means replacing the entire screen, since the digitizer and the LCD are generally built as an integrated assembly.

That’s a task that either requires professional help or serious technical skills, as you can see in iFixit’s screen-replacement guides for the iPhone 5 and the Samsung Galaxy S5. And as Wiens said, if the repair isn’t done right you can still wind up with a degraded digitizer.

If the digitizer on an old phone starts to lose its mind, you’re better off cutting your losses and getting a new phone.

Tip: iOS 9’s tiny Back button

One feature in Apple’s iOS 9 that I’ve been using increasingly often doesn’t even appear in Apple’s list of new iOS features: a small “Back to…” button that appears at the top left corner of the screen when a link or function in one app sends you to another app.

For example, if I tap a link in an e-mail message and that page opens in Safari, you’ll see a “Back to Mail” button in that corner. Tap it, and the Mail app replaces Safari on the screen.

That button is so tiny it’s easily missed, a point the Nielsen Norman Group makes it in its critique of the feature. But it’s still a major improvement over the old routine of invoking iOS’s recent-apps list with a double-tap of the home button, then selecting the previous app from that list.

It also represents yet another chapter in the long history of Apple and Google borrowing from each other, and in this case it’s an entirely sensible imitation.

Rob Pegoraro is a tech writer based out of Washington, D.C. To submit a tech question, e-mail Rob at rob@robpegoraro.com. Follow him on Twitter at twitter.com/robpegoraro.

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