After taking down one of those cantilever "invisible fixing" Ikea LACK shelves, I discovered that there were twice as many holes in the wall as required and all of them were enlarged and ruined (the wallplugs were very small and cheap). The wall is an interior one and it is a fairly thin coat of plaster over what appears to be fairly soft cement-like material. The underlying material around and between the holes is so damaged you can pick it out with a finger.
This means that at each end there are eight very closely spaced holes and a group of 4 in the middle. The wall is now unusable for shelf fixing in this area as there's hardly any plaster left, and the plaster around the area is flaking off:
I know I could clean the holes out, fill them with filler, sand and paint (the room will be repainted anyway soon), but I am concerned that this will leave fairly large hidden weak spots in the wall. Is it better to chop out the affected areas of wall, back to solid material and fill in with something stronger than filler?
EDIT: It's a 1930's house, and I suspect the wall is original. I haven't found any brick so far but there is some hard grey cement-like material behind the plaster that seems pretty solid. This is the best photo I can get down the hole after raking out the worst of the loose material (the "island" between the four leftmost holes collapsed, leaving one big one!)