2D: boxes getting caught on edges between tiles.

This has been mentioned before, but it seems an issue with box2d that if you slide a box along the top of a line of boxes you sometimes get snagged on a vertical edge - see http://www.iforce2d.net/b2dtut/ghost-vertices
It's a bit of a pain when you want to build levels from blocky prefabs.

One solution might be for Unity to have an auto-combine feature that combined overlapping/adjacent colliders into 1 big object.