There is someone I can very clearly think of that I have a negative Gestalt for. At first, I just had a neutral impression of the person as would be the normal or right thing to do. But after spending only a short amount of time with this person, I quickly realized who he really was. I had first thought that he was a clean, respectful person. Never have I been so wrong in my life. He constantly leaves messes in the kitchen, dishes in the sink, and uses my cooking equipment and leaves it dirty. Now if this only happened occasionally I could care less, but this happens almost every single day. In addition to this, said person is also quite disrespectful. Using food that does not belong to him without asking, and constantly trash talking any food that I make. Essentially, the more and more this type of thing happened, the worse and worse my impression got of him. In terms of the negativity effect, I suppose you could say this very quickly gave me a horrible impression of this person. Sometimes negative Gestalts can be wrong, but after repeatedly experiencing these events over and over again I would say they are really accurate.
Now on another note, I have a friend of mine that I immediately formed a positive Gestalt for once I had met him. He is one of the nicest, most respectful people that I have ever met. While at first it may have been a positivity bias that influenced my impression, it quickly became clear that it was no bias, but it was just who the person really was. My experiences with this person were just pleasant and positive one after another. I can truly say I have not had one bad experience with him, and I am happy to call him a great friend to this day.