I'd comment but I'm not allowed :(
I usually have had that problem when either the file or the line is corrupted or has Ctrl characters or in someversions of Unix if there was a space in the wrong place
try :set list and make sure there isn't a Tab, missing Line Feed or something
There's a way of viewing the Control Characters that don't display in :set list but can't remember it.
Shove comes to push delete the line and the one above and below. Save and retype it andsee if it fixes it.