A student once had a recommender who missed the deadline to submit the letter of recommendation. Despite numerous reminders, it just didn't happen. As a result, the student's application was considered incomplete and they were not even given a proper review by the admissions committee.
One horror story is when a student's entire application got deleted right before the deadline due to a technical glitch on the college's application portal. They had to scramble to redo everything in a matter of hours.