In Donald Knuth's Literate Programming, there was if I remember correctly a graph showing the evolution of TeX's number of bugs over time. This graph has remained flat for the past decade or so, suggesting that TeX might now be bug-free.
I would like to use this graph to illustrate the importance of bug-tracking software. Is it downloadable from somewhere?
The graphs I think you are referring to are in chapter 10 of Literate Programming (Knuth, D. E., 1992, Center for the Study of Language and Information) which is a reprint from Knuth, D. E., 1989, The errors of TEX. Softw. Pract. Exper. 19, 7 (Jul. 1989), 607-685.
I have not seen the graphs other than in book form but an updated list of errors is in a PDF at http://tug.org/texlive/Contents/live/texmf-doc/doc/english/knuth/errata/errorlog.pdf. Whereas the list in chapter 11 of my copy of Literate Programming covers 1978 to 1991, the PDF extends this to 2002. If you have installed TeX Live another version of this file, up to 1995, will probably be on your system as knuth/errorlog.tex.gz.
A PDF of a note on the list is at http://www.tug.org/TUGboat/Articles/tb10-4/tb26knut.pdf, a TUG conference keynote address from 1989.
As all the errors are numbered it could be a quick manual process to produce a rough (as the list is not in strict numerical order) graph by, for example, month. The wider range and content of all the graphs in the chapter would be a longer undertaking but perhaps an interesting programming exercise (the format of the TeX source may be much easier than the PDF for this).