What people don't seem to grasp is that self-assessment calculations are based ONLY on what you write on the form. If you make a mistake with the figures or leave something off, the forms are dealt with on a "process now- check later' basis. So if you get a refund you shouldn't have, when the form gets to the "check" stage you WILL be asked to pay it back. They have 6 years to ask for it back as well, and once you go into a new tax year you will start incurring interest...
SELF assessment means exactly that, and the onus is on the individual to speak up if it is wrong.
Aside from the Tax Credit fiasco (which was caused by EDS & the computer system), IME most "mistakes caused by the Revenue" are down to people not supplying the right info/ not filling in forms fully/ not understanding what part they play themselves... Like "I moved 3 years ago" (did you notify them? no), "I changed jobs 2 years ago & I'm still on emergency tax" (did you hand your P45 to your new employer? no) etc etc. I think Furball's comments are unfair.