Sandcastle Rule: If this structure was made of wet sand, would it break?
There's a part in the production process for sandstone 3D printing during which the model is fragile and brittle. It's basically like wet sand. When you design, ask yourself this question: if I made this out of wet sand or brittle clay, could I lift the design without it breaking? If the answer is "no," then your design might break in production.

Hanging structures are not printable
Any structure that has a hanging structure is like to break. This includes the end in an unsupported, heavy object, as pictured. Another example of something that would break is a miniature of a person holding a guitar with one frail arm straight out. The frail arm would break.

Exposure to water will fade colors
The colors we use are similar to Inkjet Printer coloring, which fades when wet. The same principle applies to 3D printed colors. In this photo, the original color was a sky blue (as represented in the lower right of the photo), and after we dipped the model in water the blue faded and whitened. This model should not be designed for applications that requires the model to touch water.

The more supported, the thinner the structure (& vice versa)
Supported structures are structures that connect to other structures. For example, think of an empty box. In this example, the bottom of the box is supported by the side walls. On the other hand, fan blades are not supported except in the middle. Fan blade-like structures should be thicker to avoid breakages during production.