How would you fix this?

[jteneyck]

Looks to me that the left trim piece is probably splayed out, creating that corner gap.
Probably came loose when it was dragged sideways at one time or another.
Lay that cabinet on it's side and take a thin blade (thinking hacksaw blade) and clean the buildup/crap/debre out of the open gap and pull it back together with a long clamp or two. Add glue of course.

I think this is the likely cause, although both pieces could be loose.  The only way to know is to turn the whole thing over and look.  If that's the problem and you get it cleaned out enough to pull the corner back together, I would added a block or triangle in the corner, behind the joint, and glue/screw it to both trim members when you glue the corner back together.  

John

[Paul K. Murphy]

I’d Bondo it, and then spot-finish.