I have got a spreadsheet for doing separator vessels and it has a least one error in it.
I refuse to let my young engineers use such spreadheets until they have done the calculations by hand. The aim of this is to get them to think about the magnitudes of the results before hand.
The problem is computers and the internet. There are good things, such as this site particularly the discussion and disemination of information from some of the MVP's. BUT no one has to think about things any more. People can download programs that may be wrong or that are not appropriate in that situation.
I always run the hand calcs first to see if I can depend upon the software calcs. Sometimes the software calcs results force me to revisit my hand calcs and lick wounds there. I find it a good check and balance, and it gives me a good "sense" of the import of what the software calcs are telling me.
But, all in all, I would hate to have to do all those calcs by hand (or slide rule) as I used to do.