Agreed. Additionally, your body doesn't know you "missed a rep", it's not like your body will go, "HEY! You missed that rep. Therefor you'll get no results unless you do something extra." Additionally, your body isn't a machine, as much as people like to think so. I feel like saying, "I will do 'x' number of sets and reps with 'x' weight" is a good frame work to start your session, but it's kind of like saying, "I want to get to 'x' pounds on the scale." Kind of arbitrary and you shouldn't be so attached to it. Every session is different, and a smart lifter will work AROUND a problem, not barrel through it.
If you don't get the additional rep you intended, as @Chris_Colucci, ask WHY you didn't get it. Some days are different than others, you might always nail a certain weight on a certain exercise, one day you go into the gym and feel strong as hell, other days warm up weights seem heavy.
If I'm aiming to do incline dumbbell presses with 100 pound dumbbells and am aiming for 8 reps, and achieve 7, you can bet your ass I'm not about to add more work or "punish" myself with more volume. I'll say to myself, "alright, I got 7 reps, that's still pretty good. Maybe I warmed up too much...maybe I wasn't focused...or MAYBE I'm just not strong enough yet for that 8th rep. Let's see if I get it next time."
Also, if you missed a rep, obviously your body is telling you something is different than what you expected, so missing a rep then adding MORE work doesn't seem like the best idea.
I would also ask, who cares if you miss a rep? I mean unless you're getting ready for a weight lifting contest where you have to perform those reps with that weight, does it really matter that much?
No I don't think so. That one rep makes no difference, and your body won't "get used to handling weight for reps" with that method.