The 'Guide to the Expression of Uncertainty in Measurement' (GUM) has been in use for more than 20 years, serving its purposes worldwide at all levels of metrology, from scientific to industrial and commercial applications. However, because it presents some inconsistencies, both internally and with respect to its two later Supplements, the Joint Committee for Guides in Metrology, which is responsible for this document, decided that a major revision of the GUM should be undertaken. In the present contribution, I present the motives for revising the GUM and the reasons why a draft of the revision was not well received by the metrological community.