Fairly early on in my career I was on a team writing a GUI programming environment (think old school flow charts) that generated ASP (classic) or Java backends.

Fairly early on in my career I was on a team writing a GUI programming environment (think old school flow charts) that generated ASP (classic) or Java backends. We allowed simple expressions in assignments with the basic arithmetic operators and functions. We parsed them into a tiny AST and output Java or ASP code for them with lots of explicit parens and casts. After fiddling with minor differences in precedence, associativity and implicit casts we still had some cases that did not match. Pairing with a more senior dev we wrote a C program to generate and compare a large number (tens of millions) of random expressions trying to explore the important variables (small and large floats. Floats that cause funny rounding. Long sequences of almost the same precedence etc). We got all three languages to agree to full precision for almost every case but in each run there were stubbornly alway a few dozen we couldn’t get to match for non obvious and seeming random reasons. This was a prototype that never saw real production use and we spent a couple of weeks on it… it always bothered me that we never understood that last 0.00001%