It could be that you have an older version of Open Office.
The auto correct feature has a box labeled "Replace dashes" ; when that is checked on the older version of OO, it replaces it with a big long dash. At that point, spell checker gives an error. If the box is unchecked, it didn't give an error.
On another computer, I have Open Office 1.1.0 and that didn't give the error even when the "Replace dashes" box is checked.