I have found that the European airlines are notorious with honoring seat assignments. One time on KLM for AMS-ORD, they cancelled my elite seat assignment (was Gold on NW at the time) in the front section of the 747 and reassigned me to the rear of the aircraft. They lied and said that the flight needed to be light upfront for weight reasons. I boarded and found someone in my original seat so I just took a seat in the front middle row. SAS also cancelled our seat assignment for CPH-EWR once.