I met a guy who mixed 23 and 25 mm for his front and rear tyre respectively.
Now thinking about it, I believe it's a pretty good idea since the rear wheel bear a higher proportion of the rider's weight. Wider rear tyres would support more weight with the same pressure as the front. It also helps reduce the wear and tear on the tyre itself.
So why don't bicycle (especially racing) run a slightly wider tyre in the rear, as the rear wheel mostly bear the weight of rider? It makes the ride more comfortable, and not aerodynamically terrible as the front is still 23 mm.