Re: the suggestion about changing the layout margins "before" and "after"
It turns out that I was wrong about the suggestion of varying the layout margins; it seems that I misunderstood what the layout margins were affecting. The layout margins control the beginning and end of the current layout, and since my simple document had just a single layout, a change in the "before" layout margin was offsetting the text vertically enough to sometimes avoid the bad pagebreak that cut off the line of text.
Further experiments show that with exact linespacing, all that matters is the true height of the text box. With exact linespacing, the number of lines that will appear in the page's textbox is very stable; the same number of lines appear on every page.
If a line of text is partially cut off, what one needs to do is to make adjustments of the document top and bottom margins so as to get the entire line of text to not be cut off. As long as a standard double-spaced (24 pt exact linespacing) page with no nonstandard linespacings for display equations or headings will display properly, then most of the time the pages will be paginated properly. However, as John may have suggested, it really comes down to a given page's text length. Certain pages will have the last line cut off if the total text height falls in just the right range and if the last line of the text on that page has exact linespacing. If the last line has single/double linespacing, it appears that it will break to the next page properly.
To conclude, if you have to use exact linespacing because of your use of in-line equations, etc., you are going to have to look at
every page in the document to see if it has a bad break to the next page. This is clearly unacceptable, it is not due to user error, it is a serious problem in the pagination algorithm. I sure hope that Apple will fix this problem soon. I think that this is a fatal problem that rules out the use of Pages for the time being for mathematical/technical documents with decent appearance.
Sorry for the rant, but I wanted to close out my contribution to this thread to address the incorrect suggestion I made that the layout margins could be adjusted to get this problem to go away. All that matters is the text box height and your choice of leading/linespacing; you might be lucky and have things work out, but I'll bet in a long document that you will be sorry if you use exact linespacing.