Some slicers and software include mesh error detectors. These tools are handy, but they sometimes generate excessive alerts, with too high a sensitivity and thinking they're detecting problems that in reality will have no consequences when printed. 

Such an error message can therefore occur without any real impact. Nevertheless, if in doubt, don't hesitate to contact us, if possible with the precise STL on which you're encountering this situation, and we'll look into it in detail.