Undefined behavior (UB) in C++ is one of the hardest categories of bugs to deal with. It can silently corrupt memory, cause crashes far from the actual mistake, or — worst of all — just happens to work on your machine. A significant share of UB in real codebases comes not from exotic language features, but from basic misuse of the standard library: accessing a vector out of bounds, calling front() on an empty container, or dereferencing an empty optional. C++26 addresses this directly with standard library hardening, introduced via P3471R4.