In Berkeley Snap!, I've developed a "block" that applies Heron's Formula (calculating the area of a triangle given the length of the three sides). It functions, but an error is reported. Below is a screen image that encapsulates everything. (FYI, the graphic below shows three such blocks with the names HF2, HF, and Herons Formula. In an effort to solve the issue, I made all three.)
125.14 is calculated correctly by the method, but it also flags a "internal error" that the "reporter didn't mention." Nonetheless, the block does report, as you can see in my block editor box.