As a flipper, one of your most valuable skills will be the ability to use plaster tools and drywall joint compound to hide anything. Case in point, one of my upstairs bedrooms. I find that the roof leaked, and the plaster failed. I kept wondering why a corner remained damp long after the roof and gutter above had been repaired. Then I took the wall apart, and understood.
Under the surface, I found drywall corner bead, a very bad sign when you're working with plaster. Under a generous amount of joint compound, I found three layers of wallpaper, and finally the plaster. The plaster has only actually failed where the accumulated crap on top of it held the water in for long enough for it to explode. So I have more demolition than I originally understood, in addition to the anticipated plaster repair. I fear I'll end up hand-scraping every wall surface.
But, if you encounter this, and you're an evil flipper who can use a plaster float and a bucket of drywall mud according to the black art, you can hide all of it, slap on some corner bead, and be ready for paint in under 24 hours. Or you can invest an extra three days stripping it back to the plaster, repairing the plaster properly, and applying two coats of oil-based primer. I'm going to have to do that now, and then some. Had they not taken the easy way out before, I'd have far less work ahead.
I've been advised to be suspicious of any alterations made in the 70s or 80s, as in those decades the neighborhood was in severe decline and nobody was making long-sighted investments. It looks like there was another wave around 2000 of quick flips. So far, I can't tell which of those categories this particular sin falls into, but (so far) at least it appears they've only screwed up finishes, not structure, and only made silly electrical mistakes, not criminial ones.