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Abstract This paper is a case study of program evolution. The author kept track of all changes made to T E X during a period of ten years, including the changes made when the original program was first debugged in 1978. The log book of these errors, numbering more than 850 items, appears as an appendix to this paper. The errors have been classified into fifteen categories for purposes of analysis, and some of the noteworthy bugs are discussed in detail. The history of the T E X project can teach valuable lessons about the preparation of highly portable software and the maintenance of programs that aspire to high standards of reliability.
Donald E. Knuth (Sat,) studied this question.