In studying machine translation software design, computer experts and linguists have traditionally concentrated on a number of phenomena deemed to present special problems and thus require particular attention. Among the favourites in this connection are morphological analysis, prepositional dependencies and the establishment of antecedents. These and similar subjects have been dealt with at great length in the numerous papers written over the years to demonstrate the necessity of adding one or more specific processing features to the software under design or pilot development. Experience in the practical upgrading of operational systems has however tended to reveal a surprising variety of quite different problems and has shown that the fears of designers and theorists are frequently unfounded. Indeed, in tailoring a system for use by translators, many quite unexpected types of error emerge which, in the absence of sufficiently comprehensive studies, have to be eliminated largely on the basis of trial and error. The paper presents several examples of translation problems of this type and explains how difficult it can be to formalize their resolution in computer programs. Special reference is made to the English-French version of Systran, under development at the European Commission in Luxembourg. Explanations are given of the identification of error types, the human effort involved in their study, and the testing procedures used to check the validity of the action taken to reduce their occurrence in routine translation work. Finally, a number of suggestions are made for those working on design aspects of new systems in the hope that by paying lessattention to problems which have already been solved, efforts can be concentrated on the specific areas which continue to cause frustration for those required to correct or use machine translations in practice.