No, I'm saying that putting common code in a separate method is not necessarily -- as in, "as a logical result or consequence" -- less error-prone.

I'm not advocating cutting and pasting huge blocks of code everywhere, but a 1-line calculation duplicated in 4 spots is probably okay.

It probably is okay to duplicate a calculation 4 times, all other things being equal. At the same time, I'd say it's probably a sign of other problems. If you need to reuse a calculation, that calculation is part of your application's domain (using the term loosely) and should be represented there, rather than being embedded inside scenario-appropriate parts of the system.