I've given low scores / overall impression to beers that were really good, but out of style for the category they were entered in.
You've got to remember that they aren't (supposed to be) judging the beer against how much they like it, but how close it is to the style it was entered as.
The last comp I judged, we had a really great Belgian blond ale and I would have had several pints of it.... But it was entered as an American pale ale, so it got a really low score.
Not sure it if had gone bad in a good way or if the entrant just mislabeled the bottle.