In the first edition of Elliott Mendelson's classic Introduction to Mathematical Logic (1964) there is an appendix, giving a version of Schütte's (1951) variation on Gentzen's proof of the consistency of PA. This is intriguing stuff, crisply and quite accessibly presented. The appendix is, however, suppressed in later editions (in fact, from the second onwards, I learn from the comments below).
Now, a number of people have said that the appendix is one of the most interesting things about the book. I agree. I too remember being quite excited by it when I first came across the book a long time ago!
So: has anyone heard a folkloric story about why Mendelson suppressed the appendix? I've never heard it suggested that there is a problem with the consistency proof as given.
Context, if you are interested: I'm writing up a survey of some of the Big Books on Mathematical Logic that will become part of my Teach-Yourself-Logic Guide, and I've got to Mendelson. You can get the current version of the Guide by going to http://www.logicmatters.net/students/tyl/