A method and device for detecting errors in a digital video signal comprising a sequence of image frames, each image frame comprising a sequence of image slices, each image slice comprising a sequence of macroblocks and each macroblock comprising a plurality of pixels. A macroblock decoder includes an error detection unit that operates to calculate an error metric between pixel values on at least part of the boundary between a current macroblock and one or more adjoining macroblocks and to label the current macroblock as suspicious if the error metric is greater than a threshold level. The threshold level is adjusted according to a weighted average error metric from one or more previous image frames. Suspicious macroblocks and subsequent inter-coded macroblocks may be regenerated according to a concealment strategy if a syntax error is found within the current image slice.