If, in the post editor's 'CSS' mode, you enter a leading or trailing spaces, the editor tries to convert them to entities, but when posted, the ampersands get turned into & entities themselves. So, for example, if I try to make a post that says (quotes added just to highlight white space):
" piggy plops "
the generated HTML in the page is:
piggy plops
and so the browser renders it as:
piggy plops
It should, of course, appear as:
piggy plops
[Edit: It also happens with double spaces after a full stop. So "A b c. E f g" will appear as "A b c. E f g"]