Problems involving impulsive tensions in string-that is, the impulsive interaction of two (or more) bodies connected by strings-have been discussed in textbooks on classical mechanics for at least one hundred years (Williamson and Tarleton 1885, Tait and Steele 1882). These problems, in one form or another, crop up in most textbooks designed for A-level applied mathematics or for similar courses at the higher end of secondary school curricula in the British Isles. The examples studied in the various textbooks are treated in a very similar manner by most authors, suggesting a common tradition. Unfortunately, most of these treatments contain a misconception of a very basic nature, which is the subject of the article.