In Season 6, Episode 5 of Game of Thrones, "The Door," Bran delved ever deeper into his powers and visions. The most shocking moment was when he stood amongst an army of wights in front of the Night King in a vision and the Night King actually reached out and touched him. How did the Night King see Bran on Game of Thrones?
Bran has been getting pretty arrogant in his visions lately. He's always pushing the Three Eyed Raven to let him stay longer and see more without knowing what the consequences are going to be. Tonight's episode was all about consequences. In his brashest move yet, Bran entered a vision without any guidance from the Three Eyed Raven. He moved through countless wights before approaching the Night's King, who was on horseback. The first shocker was when the Night's King turned to look at him, and everyone else followed – no one is supposed to see Bran in his visions, and so far no one has been able to. Then he reached out and grabbed Bran's arm.
Back in his tree fort, Bran's arm still bore the mark of the Night's King's hand. It was going to cause real problems going forward, but seriously – how did he see Bran in the first place?
It's not entirely clear. Bran's visions so far have all been of past events, but there's no way to know definitively that he was seeing the past Night's King. Perhaps Bran was still in the present, he was just seeing events far outside his realm of experience, and this could be why he was able to be seen.
Maybe it is just because the Night's King has powers even the audience doesn't know the full extent of yet. Tonight's episode revealed the origin of the White Walkers (they were created by the Children of the Forest to battle the First Men) but viewers still don't know a lot about how they operate day to day, or just how powerful they really are. Maybe their psychic abilities far outweigh Bran's, so that's why the Night's King had no trouble spotting (or grabbing) Bran.
It's also possible that the Three-Eyed Raven's guiding influence was able to shield Bran so far. Perhaps Bran would have been getting up to even more damaging shenanigans in his visions without the Three-Eyed Raven to protect him from being seen or heard. After Ned seemed to hear Bran at the Tower of Joy, the Three-Eyed Raven pulled him pretty quickly from the vision. It's possible it was always a danger.
Whatever the reason, Bran's decision to go warging while flaunting the warging buddy system has cost him two of his dearest friends (Hodor and Summer), as well as his mentor. He'll need to get a handle on these visions soon if he wants to avoid causing so much damage in the future.