A small measurement error can travel surprisingly far through a process.

A small deviation at the start of a process does not stay small. Each subsequent step takes the flawed input as fact and builds on it, so the error is carried forward, sometimes multiplied, and always laundered into something that looks like a solid number by the time it reaches the end. The final figure carries no memory of how it began.

When a result matters, trace one number back to its source rather than checking the arithmetic in front of you. Ask where the original measurement came from and how much slack it had. The most valuable checking usually happens closest to the beginning.