If for some reason I'm typing on the line before a paragraph, the word wrap feature *sometimes* automatically merges them.  It turns out this is almost never what I meant to do.  It seems that the trigger for this is:

1.  Type something.
2.  up-arrow to previous line.
3.  Start typing.

Reason this bites me is that often the "something" in the first line is something signature-y.  So I might write something, then write "- seebs" at the bottom of it, then think better and add a new paragraph -- but then when I hit line wrap, that gets merged.

Not sure what the "fix" would be.  This doesn't happen in some other cases; I'm assuming it's a general case of "moving within a paragraph doesn't break paragraphs", in which case, maybe if you move up from a paragraph to a blank line before it, this should be considered a separate paragraph, not moving within a paragraph.