We tend to see so many pairs of things: pair of sconces, pair of table lamps, pair of end tables, pair of nightstands, pair of chairs, pair of mirrors...I love symmetry and balance but once in awhile it's fun to mix it up a bit.
Dining chairs are a good place to mix 'n match.
Mixing and matching mirrors (especially ones in a similar frame or style) but of different scale and sizes (not unlike a post I did on a whole wall of mirrors).
Again, different mirrors in the bathroom, mixes up the standard pair of mirrors.
The casual look of pulling up several "mixed use" chairs for dining.
Matching but mixing ever so settle, the difference in the seat backs around the table.