LaTeX tries to guess whether a period ends a sentence, in which case it puts extra space after it.
Here are two examples where it guesses wrong:
I watched Superman III. Then I went home.
(Too little space after "Superman III.".)
After brushing teeth etc. I went to bed.
(Too much space after "etc.".)
Note that it doesn't matter how much whitespace you use in the LaTeX source since LaTeX ignores that.
Best Answer
I found the answer here: http://john.regehr.org/latex/. Excerpt:
When a non-sentence-ending period is to be followed by a space, the space must be an explicit blank. So the second example should be:
The converse of this problem happens when a capital letter precedes a sentence-ending period in the input, as in the first example. In this case LaTeX assumes that the period terminates an abbreviation and follows it with inter-word space rather than inter-sentence space. The fix is to put "
\@
" before the period. So the first example should beA handy way to find this error is: