I've only ever had partial floods - more an enthusiastic leak actually. And every time it has happened it has been my fault, forgotten to tuck the thumb loop back inside the suit, changed seals (silicone) and not checked they were properly seated and so on.
My last leak was when I fitted dry gloves and slightly misaligned the wrist seals, I noticed a slight trickle during the dive but nothing else. When I got out I had a pint or so of liquid in the arm of the suit. Didn't know it was that much and I wasn't cold, it hardly affected the dive. The water was about 9 degrees celsius so it was cold both in and out, but I wasn't.
So as others have said drysuit leaks do occur but they are usually no big deal. Take your time and set up carefully and you will not have a problem - P