I had to explain recently that `a * x !== b * x when a !== b`*... it is infuriating hearing "but the result is the same in this other competitor" coupled with the "maybe the problem here is you're not knowledgeable enough to understand".
We've definitely had our fair share of "IDK what to tell you, those guys are mathing wrong".
TBF, though, most customers are pretty tolerant of explainable differences in computed values. There's a bunch of "meh, close enough" in finance. We usually only run into the problem when someone (IMO) is looking for a reason not to buy our software. "It's not a perfect match, no way we can use this" sort of thing.
At this particular job I used the plus, the star (multiplication) and once I even got to use the minus.
There's a legend going around that a friend of mine has used division, but he has a PhD.
Sometimes. I have often had to say "no" because the customer request is genuinely impossible. Then comes the fun bit of explaining why the thing they want simply cannot exist, because often they'll try "But what if you just ... ?" – "No! It doesn't work that way, and here's why..."