In my C++ course, I require that the code runs correctly under 5 different compilers (GCC, LLVM Clang, Apple Clang, MSVC, Intel C++ Compiler Classic) on 3 OS (Ubuntu, Windows, macOS) in Release, Debug+Valgrind, Debug+Sanitizers modes, and students still get UB quite often.
Extra reading: http://evan.nemerson.com/2021/05/04/portability-is-reliabili... and discussion at https://news.ycombinator.com/item?id=27044419