Type:

  { echo foo; sleep 2; } | less -F

then Ctrl-C. One gets the "less" prompt, i.e. "less" doesn't exit,
while the description of the -F option is:

  Causes less to automatically exit if the entire file can be
  displayed on the first screen.

Obviously the entire file (the string "foo") can be displayed on the
first screen, so that "less" should exit.

Note: I automatically pipe some commands to "less -+c -FRX" in order to
use the "less" pager when need be, so that this bug is a bit annoying.