True story of how an interview exploded for me:

I'm asked to code a simple matrix in java. I ask about the fill factor, and get told quite low. The constructor takes the dimension and it's assumed square. I ask the interviewer if a csc matrix is ok, and he kind of says he's not sure what a csc matrix is. I tell him it's column sparse, and proceed to quickly write out, on a whiteboard, a simple csc matrix -- basically, an array of dimension columns of ArrayList of tuple <index, value>. The constructor is pretty much

   ArrayList<Tuple>[] columns;
   void SparseCSCMatrix(int dim){
      columns = new ArrayList<Tuple>[ dim ];
   }       

The interviewer then starts going at me because having, eg, a million columns requires an array with a million elements in it. I say yes, duh. He says it's quite inefficient -- what if I only use 10 columns out of the million? My response is, well, that's fine -- but what you're describing isn't a matrix. It turns out that what he really wanted was a HashMap from tuple: (x idx, y idx) to value, and that a typical load would be perhaps 1k elements in a space of 1 million by 1 million.

We then go at it about whether I should have asked him to clarify, and I basically decide I don't have to play along with stupid gotcha questions so I told him words have meanings and a "matrix" with only 10 non-empty columns is not a matrix. And I don't have time for wankers.

At least I learned what they were like fast...