Playing around with the ITA Matrix and I noticed that I couldn't find any flights between London and Havana, nor between Miami and Havana, Mexico City and Havana, or Madrid (Barrajas) and Havana. I searched for a one-way trip, with no restrictions on the number of stops, within a month of 1 September. I even removed the restriction that it only return flights with available seats.
This made me wonder if maybe the problem was Havana itself. Does ITA Matrix work for all destinations, or are there limitations? (And is Cuba one of those non-functional places or am I just Doing Something Wrong™?)