Since 3D printing is a new technology, there may be problems that we don't know about. ABS sometimes gives off fumes when being printed because of the heat, but seems to be stable afterwards. Other materials like PLA can be food safe. But there are a lot of variables; the object could be dipped in acetone to smooth the surface, or there could be other additives mixed into the materials. The more common problem is that 3D printing processes may not create a completely smooth solid surface, so germs can find nice places to live. Currently, the only 3D-printed materials considered food safe are glazed ceramics and polished metals such as stainless steel. For printing food, chocolate, frosting, and so on, you need to make sure that the materials and the printer itself are food safe.