Yes, mine rocks when placed on a flat surface, it was the first thing I noticed. At the time I quickly looked at the bottom and assumed it was because there were three "feet" on it (only one in the middle at the bottom), but trying it just now I see it only rocks in one direction (top right / bottom left) so it must be something oddly sized inside causing the casing to be deformed when it was screwed down.

I guess a solution would be to put small pieces of tape over the feet to pad them out, but it might foul against the slide if a few pieces are needed. I have only used on a flat surface a couple of times so it probably won't be much of a problem, just a niggle really.

Gareth