The impact of H.264 artefacts on subjective quality is still under investigation [1]. Contrary to existing approaches, this paper considers the impact on perceived quality of real H.264 artefacts in HDTV videos. A method for the design of spatio-temporal classification is proposed. This classification is used to locally distort sequences with the H.264 codec. Class-generated sequences are then subjectively assessed in order to evaluate the importance of each spatiotemporal class. An attempt to find a relation between local and global quality loss is then presented and discussed, along with an annoyance function model.