I made a typical mistake and used a hand router to route in the wrong direction. The router moved away from my straight edge and curved through the wood.
Generally, when hand routing, you want to move from left to right, so that means counter clockwise on the outside edge and clockwise on an inside edge (like inside a picture frame).
The router bit spins clockwise – if you route the wrong way the bit acts like a wheel running along the edge and pushing away from your edge as the blade of the bit cuts into the material.
If you are routing dados and only have a straight edge (and not a jig that will hold the router centered) you also need to route from the left end of the straight edge if the router is between you and the straight edge.