Hello,
If the number of visible files in the directory the luatex.fmt file is stored in is equal to six or lower, some strange overfull boxes are reported by LuaTeX, and the text is typeset as described above.
I tried to reproduce the phenomenon and had the following results (with TeX Live 2007 on Mac OS X Intel): • With the LuaTeX beta version and a standard configuration I get extremely weird output in the vein of what Jonathan observed (see FontExpandTest-beta.{log,pdf}). • I then tried to change the number of files in the directory where the format file lies like Jonathan did but could see no change. In the process, though, I noticed that setting TEXFORMATS makes some of the problems disappear (FontExpandTest-current.{log,pdf}). • Then (actually in the first place) I made the same experiments with the current SVN trunk revision and the very weird problem of the first paragraph didn't show up anymore: setting TEXFORMATS or not made no difference and whatever I tried I got the output shown in FontExpand-current.pdf. I didn't try very hard Jonathan's trick of changing the number of files in the directory where the format file is found, but it seems to me that it doesn't make any difference with my configuration. So, although the problems aren't all solved yet, it seems that some of them have already disappeared in the latest revision; and even if I couldn't reproduce the exact same behaviour as Jonathan reported, I'm pretty confident that the problem with the six or seven files in the web2c directory amounts to the same as the one I had with setting TEXFORMATS (after all it's all about kpathsea search for the format file); and subsequently, I'm strongly inclined to think that this weirdest of all bugs has been fixed in the current SVN revision. I'm also attaching the TeX file I used for all my experiments; it's only the extract from Jonathan's original e-mail but I put it here for self-containedness. Arthur