With the following plain TeX:
\hbox to \hsize {
\vbox to 2 true in {
aaa aaaaa aaaaa aaa aaaaa aaaa aaa aaaa aaa. aaa aaaaa aaaaa aaa aaaaa aaaa aaa
aaaa aaa aaa aaaaa aaaaa aaa aaaaa aaaa aaa aaaa aaa. aaa aaaaa aaaaa aaa aaaaa
aaaa aaa aaaa aaa. }
\vbox to 2 true in {
bbb bbbbb bbbbb bbb bbbbb bbbb bbb bbbb bbb. bbb bbbbb bbbbb bbb bbbbb bbbb bbb
bbbb bbb bbb bbbbb bbbbb bbb bbbbb bbbb bbb bbbb bbb. bbb bbbbb bbbbb bbb bbbbb
bbbb bbb bbbb bbb. }
}
the goal is to see two boxes side containing a paragraph or so of text. Except that when TeXed, the first vbox stretches to the full size of \hsize, squeezing the second vbox off the page.
Why doesn't this work as expected? There might be a better way to implement side-by-side paragraphs, but I'm still interested in what's wrong here.