I had forgotten what this effect was called, I searched for it all evening
Bystander effect
"A victim may be able to counter the bystander effect by picking a specific person in the crowd to appeal to for help rather than appealing to the larger group generally. This places all responsibility on that specific person, instead of allowing it to diffuse"
And my favourite
Somebody Else's Problem field (SEP field)
"Simply put, an SEP field is an energy field similar to an invisibility field. When somebody, or something, is surrounded by a SEP field, the human brain perceives it as "somebody else's problem", and therefore will be incapable of paying attention to the object (or even seeing it, or recognizing its existence) unless it is being specifically looked for. A primary example of this was given in the third book Life, the Universe and Everything, when a UFO lands in the middle of a cricket ground during a match, and the assembled crowd completely ignores it. Another prime example occurs when the aforementioned ship's field is extended so that the characters fail to notice the fact that they cannot breathe or the fact that the asteroid that they are standing on does not have enough gravitational force to hold them down."