Bad RAM slots

Ok so i'm working on a computer for a friend and running the memtest 86 off the hirens cd and i find that both ram sticks test bad so i check the slots with ram that is good and they still test bad. obviously this means bad RAm slots but my question is how can a computer turn on and run fine with a bad ram slot??? this confuses me i mean ive seen weird shit but if the slots are bad then the computer should not start up am i right?? the POST test shouldnt even pass to begin with right??