You expect a few bugs in a new operating system, but a busted calculator? That's... irritating.
But that's exactly the situation in iOS 11. When you rapidly add two or more numbers, Calc doesn't always register the third calculation press -- which could result in some serious mathematical errors.
Try it: Tap 1+2+3 in rapid succession. You'll very likely end up with 23 (or 24 if you proceed to tap "="), because that third "+" tap wasn't captured.
Why? There appears to be some "animation lag" at work: When you tap a number, it briefly lights up and then fades. But after the second number, if you try tapping a function (plus, minus, multiply, etc.) before the number has fully faded, it won't register.
The solution: slow down. Yeah, it's a crappy solution, but it does work.
There's anecdotal evidence this problem has existed since iOS 11 was in beta; my iPhone is running iOS 11.1. One of my CNET colleagues installed 11.2 (which itself is still in beta) and found that Calc seemed to be working better, but that the bug wasn't completely fixed. "Inconsistent at best," she said.
Needless to say, until Apple addresses the problem, you should slow down your numeric data-entry or switch to a third-party calculator. (Already using one? Share your favorite in the comments!)