Dangling pointers, double frees and the like, mostly.

Those are bugs you wrote in. UB is not the problem. Your code is the problem.

Tell me you haven’t run into those and I’ll laugh in your face and call you a liar.

I ran into bugs. Do you understand that UB is not the problem if you're pushing broken code? It's not the C++ standard that's messing up if you're writing in use-after-free bugs.

The irony of your comment is that some implementations take advantage of UB to prevent programs from crashing and actually continue to work in some scenarios such as use-after-free and even dereferencing null pointers. But that's not caused by UB, is it? Those problems are caused by developers like you and me who didn't knew what they were doing and even failed to either pay attention to the errors flagged by compiler and static code analysis tools, or even failed to onboard one.

I mean, think about it for a second. Let's say we have a magic wand that can update any C and C++ standard version of your choosing, and we specify that each and every single instance where behavior is left undefined is updated to specify that the program should automatically crash. Awesome, no more UB. What does this mean for your code? Is it now bug-free? Is it now working well after crashing all the time due to the code you added? What role did UB played in this mess?

Do you understand this?

I repeat: detractors just parrot undefined behavior as some kind of gotcha in ways I’m not even sure they fully understand.