That's a paragraph border triggered by AutoFormat As You Type. The feature
has applied a bottom border to the para above the line... Easy to fix
Click in the para above the line, go to Format> Borders & Shading, on the
Borders page click the None setting on the left, then OK.
In the future you can do one of several things rather than the above to:
A) Remove the border immediately - either Undo (Command+Z) when it happens
or use the Lightening Flash button that appears immediately after it occurs,
or
B) Avoid it happening altogether by going to Tools> AutoCorrect> AutoFormat
As You Type, clear the check for Symbol Characters (--) with Symbols (‹).
HTH |:>)
Bob Jones
[MVP] Office:Mac