I hung 6 Ikea shelves (these ones, which are metal L-shaped brackets with wood shelves on top) on plaster walls using medium-duty plastic anchors.
It was a real issue while we were working. The building was built in the 60s and the wall I'm using appears to be entirely plaster with no studs, no lath, nothing (as far as I can tell). Just solid plaster all the way through.
On the 6h shelf, we thought we'd found a stud and didn't use an anchor. Turns out whatever that was it wasn't a stud. 5 of the 6 are still up after about 2 weeks, but the 6th has fallen.
Ideally I can put the shelf up in the same spot. What I would like to do is fill the holes with spackle (or something else?) and push a masonry anchor into it before it sets, then re-hang the shelf. Is that insane?
If it is insane, can I just cover the holes, move the shelf over an inch, and try again with masonry anchors this time?
Edited to add some pictures: