I have the files for The Preppy Lion books in a folder on my hard drive that’s gone through moves from who knows how many computers since around 1990ish and I found a letter to an editor at Springer-Verlag from November 1992 with this gem of a sentence:

Because LATEX 3 is likely to appear in approximately two years, the latter volumes are currently at a low priority.

Also, loads of old stuff for volumes 2, 3 and 5 that I’d forgotten about.

I had a bit of a shock while looking over a section of my book on typesetting special characters to discover that my example of how to typeset a four-dot ellipsis in LATEX was coming out like:

when I was pretty sure it was supposed to look like:

 It was pretty easy to discover that this was because in XeLATEX (and LuaLATEX), \ldots was no longer typesetting spaced dots but using the Unicode … character instead. The appearance, as evidenced above is quite different and the Unicode character doesn’t compose nicely with a following period (as required by some styles, most notably MLA, but some others as well—the example made it into the book originally because it was repeatedly asked about when I was teaching LaTeX classes back in the 80s/90s).

With some digging into source2e.pdf, I turned up \UndeclareTextCommand which lets me undo this. It’s just a matter of adding 

\UndeclareTextCommand{\textellipsis}{TU}

to my document class to get the intended result and my ellipses are returned to normal. Unfortunately, I’ll also need to include a mention of this into a sidebar for those who might encounter this edge case on their own.

Update: While writing the text for the book, I discovered that the sequence …. came out almost correct in Monotype Ehrhardt (the spacing is still a little uneven, but not as dramatically so as with Latin Modern), which made it a little complicated to write the sidebar. I had to switch to Latin Modern for those ellipses to illustrate the problem. It looks fine on the blog too. Latin Modern’s ellipsis is broken!