Notes

Code the C++ standard places zero requirements on. Not “probably crashes” — the compiler is legally allowed to do anything, including appearing to work perfectly.

int arr[5];
arr[10] = 1;   // UB - no bounds check, no guaranteed crash
int* p = nullptr;
*p;             // UB - might crash, might not
int x;
x + 1;          // UB - reading an uninitialized variable

This shows up constantly across this vault: Pointer arithmetic, Memory leaks and dangling pointers, and std_vector’s reallocation gotcha are all specific instances of undefined behavior.

Why “might work fine” is the scary part

A UB bug can pass every test, ship to production, and only misbehave months later when a compiler upgrade or a different machine changes what the “undefined” behavior happens to do. No guarantee it breaks the same way twice — far harder to reproduce than a guaranteed crash.

Related

Pointer arithmetic
Memory leaks and dangling pointers