The following article accurately defines and demarcates the concept of source text error within the framework of the communicative translation of scientific texts with a didactic or popularizing character, as distinguished from various other closely related concepts (such as text passages inducing the introduction of improvements, or inaccurate passages originating in a legitimate pedagogical simplification, etc.). It also advances a specific classification of those textual errors resorting to the analysis of a rich collection of published and unpublished (author-made) translations into Spanish and Portuguese of excerpts of English and German encyclopedia articles, textbooks, popularizing articles and popularizing books with scientific content. Errors that occur in source texts aimed at science education and popularization, which come up rather frequently and must be corrected in the body of the target text in a communicative translation, are here classified, in the highest hierarchical level, into factual errors, when opposed to the designation of known truth or to the proper conformity between verbal and iconic components of text (six subcategories), and formal errors, when harming expository correctness or communicative efficiency (eight subcategories). In addition, the article assesses the varying cognitive requirements imposed on the translator by the necessity of spotting and rectifying those errors in a communicative translation.