Still hungry?

If you want to hold yourself accountable for the extra calories, just go back into the day's food log and add them. You'll get the "red number warning" for going over when you total out (if you do total out, I often don't) and sometimes that can encourage people to hit the target more closely the next day. If you don't wish to record the extra calories, they've still been ingested of course. Know that you may be vulnerable to evening snacking, and adjust your calorie intake for the day to mitigate that.

(Not that I'm some winner who doesn't snack at night and never goes over. Ha! I wish! But I do need to know how far over I've gone to adjust my behavior, and it rarely works to simply keep it in mind and not record it in writing.)

I think it depends on if you were truly hungry, or maybe just eating out of boredom or habit.

If you were really hungry, and find that it happens a lot at the end of the day, you could re-evaluate how you split up your calories for the day. Maybe "save" some calories from earlier in the day so you can have an evening snack and not go over your calories.

If you're maybe just bored-eating, or doing it out of habit, you can try distracting yourself with an activity you enjoy. You can also try sipping a low or no calorie beverage, like tea or flavored water.

Some people say brushing your teeth or chewing gum after your last meal of the day discourages you from eating more.

Ultimately, you have to find out what works best for you. And, of course, there's nothing wrong with occasionally going over your calories. But, if you make a habit out of it, it becomes more difficult to reach your goal.

How many calories was it, and what weight loss rate did you ask MFP to give you?

If I was really hungry, and it was an unusual thing, I'd just eat the PB & crackers. One serving of PB is a couple of hundred calories, I don't know how many crackers you ate, but a standard serving would be somewhere between 80 and 200 calories (6-7 Triscuit are 120, for example). So, if it was that, let's say 350 calories, for both the PB & crackers?

If you're set to lose a pound a week, MFP has given you a 500 calorie daily calorie deficit. 350 calories uses up part of that, but not all. You'd still be in a 150 calorie deficit for the day. Looking at it over the week, if this was a rare thing, and you happened to come in a few calories under on other days - which I'm not saying you should engineer to "make up for it" because I think that's A Bad Plan - you could end up just about even for the week.

In that kind of zone, it's no big deal. If it was spoonful after spoonful of peanut butter, and half a box of crackers, that's different.

If it's a common thing for you, then I agree with considering whether you're trying to lose faster than needful (too low calories), whether you could get better nutrition or more filling foods or different timing of food during the day, and feel more happy/full on your routine. If so, try a different routine.

Sometimes, even inadequate sleep or high stress can cause fatigue, which triggers cravings - because calories are energy, and a fatigued body seeks energy. If that's the case, working on the root problem can help, via improving sleep quality/quantity, or using other stress management techniques.

It's not some kind of epic battle between good and evil, or success and failure . . . it's just routine analysis and problem solving to figure out the best way to stay full, happy, and well-nourished while simultaneously losing weight at a reasonable, moderate rate.

Hang in there, keep figuring this out, dialing in a good personalized routine - you'll do fine.

Yes, veggies are carbs but if you were to compare number of carbs(also calories), which would be higher/lower? Crackers, breads, pasta, rice, wraps, etc. or carrots, tomatoes, green beans, cucumbers? Lower calories in the veggie dept.
BUT with that said, veggies don't usually fulfill my snack cravings. Snacking, for me, would gravitate right to those crackers and peanut butter. Maybe, OP, try pop corn or rice cakes instead? Or a low cal. fudgsicle, or even 2. Low fat pudding or nonfat plain yogurt. And the peanut butter portion is a high fat high calorie choice. That's coming from someone who couldn't live without her peanut butter.

My main way of fitting in my preferred snacks is keeping calories out to use for them, either by switching my meals around like making them smaller, adding more walking or whatever exercise into my routine to allow me an extra 100-200 calories a day.

It might sound fairly ineffective, but having just two crackers (Ritz/club, not large in size and not overly flavored) is helpful for me. I have them 1-2 times a month either right before going to bed or in the middle of the night when I can’t sleep.