It´s really not a rendering bug, you see

Most rendering algorithms deliberately employ approximations and other shortcuts for efficiency. These economies-not coding errors-produce characteristic image artifacts. This article classifies the best-known varieties. We wrote this article in the hope of saving programmers a lot of time debugging something that is really not a bug, but instead a limitation of the algorithm. Our taxonomy is not exhaustive, but it does include many common rendering problems that we have encountered over the years. Most of these limitations can be circumvented with the proposed solutions. In reality, there are more workarounds than true solutions. This imbalance warrants more research into proper solutions to these and similar problems