One of the biggest lies told in construction is that they will lift the rebar as they pour the concrete. For the first five minutes, some will actually pretend to do this, but then they get too busy to pretend any more and they just walk on it. Chairs will cost another $100, so most don't do this to save money, it's a pain to have to step over and in between the rebar. Much faster and easier for them to not have to deal with rebar and lie about pulling it up then lying about how it will remain in the middle of the concrete while they walk on it.
Rebar has to be somewhere in the middle to lower third of the pad to do anything. That means an inch of concrete under the rebar. I prefer an inch and a half chair when using 2x4's for forms, or a 3 1/2 inch pad that puts it more in the center, but we're splitting hairs as long as the rebar is surrounded by concrete. That means there has to be chairs under the rebar to hold it up in the air.
Then the biggest thing that most will do wrong is add too much water to the mix. This makes it easier for them to spread the concrete, but adds volume to the mix which has to go somewhere when it evaporates. Once it's evaporated, all that volume is gone, and the remaining concrete has to crack. This is where you see those big cracks in concrete that happen a day or two after the pour.
People that do not put chairs under rebar are very likely to add too much water to the mix.
