Well, I was in the middle of an NCR mission where I had to kill some Legion members. As soon as I killed one I was shunned by the Legion and they became hostile towards me as expected. After a little while I became vilified by the Legion and all of a sudden they are no longer hostile on sight. I have to admit that is a bit of a game breaking bug.

I saw another user report the same issue in the PC mods section. I am using no mods, btw.

I actually saw a member on another forum report this same bug. Is there any way to fix this? It's pretty annoying.

i having a problem with the ncr fame. i use to be liked then i finished a mision for them and it said ncr fame has gone up but it went down to neutral and i keep doing misions for them but it never changes

As far as I know, there comes a point in the game where, if you dissed the Legion, Caesar pardons you for your 'crimes' and asks that you come work for him. It could be that your bad reputation was purged, but you haven't met Caesar's messenger yet?