They can and they will if you employ one that isn’t made for climbing or you’re using climbing clips in critical spots that don’t have a sufficient kN rating and you ask it to catch a large climber. Keep in mind too that (real) carabiners are rated along both their major (long) axis and their minor (narrow) axis. If you have the carabiner employed in such a way that the force of a falling climber would be brought upon the narrow axis there’s a good chance it’s going to fail if asked to catch someone. So how you deploy the biner is as important as the biner you deploy.\r