Examination of the gastrointestinal contents of museum specimens is routinely used as a method for assessing diet in a wide variety of reptiles. However, this method might be biased toward detecting prey items that are less digestible and larger in size because these food items are digested more slowly. In this study, we used fixed videography on free-ranging Puff Adders (Bitis arietans) as a comparative, data-collecting technique to assess the accuracy of the traditional method of examination of the gastrointestinal tracts of museum specimens as a measure of diet. The data-collecting method affected our measures of diet: Analyses relying on museum specimens showed a much narrower diet breadth compared to fixed videography, and measures of mean relative prey mass were more than three times larger using museum specimens compared to fixed videography. Our findings demonstrate that data collected through fixed videography and examination of museum specimens provide different perspectives of a snake's diet because of the biases associated with museum specimens. As a result, the use of museum specimens to assess diet should be interpreted cautiously and with knowledge of these biases, as the technique might only reveal certain aspects of a species trophic ecology. In particular, we suggest that the routine use of methods such as examination of museum specimens and palpation of live snakes might have led to a biased interpretation of the feeding ecology of ambush-foraging snakes.