When writing a sentence as "-my example-" the whole expression apear with a continuous line when writted de last "-", the effect desapear using command+Z leaving the desired expression.
That is a "strikeout", also known as a strike through.
It is a AutoCorrect feature controlled from the AutoCorrect Options tab. Tools -> AutoCorrect -> AutoCorrect Options dialog... it can be disabled, or as noted <ctrl>+z will reverse.
It will be a stanza with localized string for "Automatic *bold*, /italic/, -strikeout- and _underline_"
The autocorrection can be disabled/enabled by checkbox either when modifying (M) or creating new (T) text while typing. Defaults are enabled for both M & T