Can a fitness tracker have a negative effect?

From experience, trackers do not just give you calories, you have to work for them. Depending on your settings and activity level you might not get hardly any adjustment unless you bust your tail. Bottom line, if you like what you are doing and it works, do not try to change it.

I don't let my Vivofit change my MFP calories. I do add running data from my Garmin watch to MFP. Just use your Fitbit or Vivofit to encourage you to hit a daily steps target.

I guess Person1 hasn't been using MFP how it is intended because Person 1s are supposed to eat back their exercise calories.
Trackers just make those measurements more accurate (in theory).

I see your point and I do think if you didn't eat back 'ordinary activity' calories before (and who does) but now your Fitbit is adding calories for it, you'll probably eat more with it. I did.

But in your example it was 'workout calories'. And the assumption seemed to be that the Fitbit calced them higher than the person. I don't know how common that is, given how many posts I see about MFP's database exaggerating exercise calories.

Your hypothetical seems to be that Fitbit would add calories beyond what allows for weight loss. IME, Fitbit is pretty spot on at estimating my maintenance calories, and thus that doesn't seem to happen.

When I first started I had a Fitbit, but it didn't cross my mind to eat back the calories, even though I saw them. I was more focused on eating a particular way and meeting my MFP calories (which I did not yet quite understand included exercise legitimately, rather than "maybe if you want to and are hungry"). I also lost at a higher rate than MFP predicted, because I'd said I was sedentary and I was not. Eventually I gathered the connection between Fitbit calories and activity level and started eating back some of them, and now I use the Fitbit more to keep me honest about my activity level (I try to be active, but I have negative adjustments when I'm not). Anyway, so arguably I did eat more with the Fitbit than without it when I recognized that the calories it was adding were connected to something real--that I was at least lightly active--but had that not been the case, had I been losing only what MFP said I should, I would have continued to ignore the Fitbit calories, I'm sure. So I'd say the effect of Fitbit was to make me use MFP properly, by correctly estimating my activity level or otherwise accounting for it (which you are already supposed to do with MFP, but many people just assume sedentary).

The bigger effect of Fitbit for me is motivating me to be more active in my daily life, so that I actually was lightly active and am often active vs. sedentary or close to it, even apart from intentional exercise.

I personally would adjust if I weren't losing weight, though, and based on the discussions I've seen here and on my friends' feed, I don't think that's uncommon at all.

My Fitbit automatically deducts over 200 calories from my daily total in the morning and I have to earn it back. This motivates me to move around a lot more to get those calories back,

I use a different brand of tracker (LifeTrak) and what I do is to take an average between my net calories there and what's calculated here.

(For those who don't know, net calories = total calories burned during workout - what calories you'd burn without any activity)

In my case, my LifeTrak says that when I'm sedentary and awake, I burn about 90 to 105 calories an hour.

I got a fitbit, but I don't sync to MFP. It tells me I burn about 3000 calories a day, and that really doesn't account for my weight lifting. So in theory, according to my fitbit, I could lose eating at about 2500 calories a day. But, I've only had it a few weeks, and twice now I've hit 10,000 steps while driving, and once while I was washing my face in the shower. I would likely gain weight because it's over counting steps.

However, I still find the device useful because it still inspires me to get more steps, even though I know the count isn't accurate. On days that I do cardio, I try to get more than 15,000 steps, and I know I'm making better decisions just so that I can get more steps (I pace while waiting for the train or brushing my teeth, take the stairs, or park far away in parking lots).