Later in life, Mr. Collins and Elizabeth will have a torrid affair.Elizabeth famously tells Darcy, "I had not known you a month before I felt that you were the last man in the world whom I could ever be prevailed on to marry." At the end of the book, he's the last man in the world whom she is prevailed on to marry. Sounds like perfectly straightforward foreshadowing, right? Well, earlier in the book, she also told Mr. Collins "I am the last woman in the world who could make you [happy]." Draw your own conclusions.
Mary and Mr. Collins were made for each other.Mr. Collins is just too dense to realize this. (Some screen adaptations follow this logic, having Mary ineffectually pursue Mr. Collins.)