As a Tester myself, I cannot understand why this is not covered by either unit tests or behavioral tests. Clicking dialog buttons in rapid succession is what we (should) do once in a while. Especially in core functionalities such as the login screen. It's one of the first screens you see as a tester. And you have default usernames, be it enabled or not.

For example, I do not own an iPhone, but at work, I made a bet with my colleague (jokingly) that I could break _something_ on his phone in a few minutes.

I did not have his finger print or pin-code, so I was very limited, I even joked "I don't need that, give it here!"

Finding out I only had a hand full of options, I focused on the emergency dialer. As any good tester would be curious about, I wanted to check the max field length, so I entered digits, copy/paste it a few times, copy/paste that string, ("wait, no limit? Not even at 1000? why?") and so on, until I noticed the interface became laggy, so of course, I kept going.

Boom, suddenly back at the login screen, tried to open the emergency dialer, but got a full blank white screen, in the meantime the phone started heating up substantially. Since it was a new Phone (iPhone 7 with iOS 10.x I believe) and the dev getting nervous, we decided to reboot it. That fixed the issue. (Curious if this is still an issue in iOS 11.x)

TL;DR: As a tester this simple curiosity should be in your blood, and especially covered in behavioral tests when your software has been around for 5+ years.