This is a bit of a tangent, but, I don't use Google sheets, only Excel, except one time a friend who needed to use a workbook of mine, did not have Excel. So I sent her the workbook and she opened it in Google sheets. Testing on my end, I opened it in Google sheets also. To my shock one of the formulas was not producing the same value as Excel.

I spent quite a bit of time with this. I entered a new formula in a new cell in Sheets, reproducing the mysterious failing formula, and it failed also. There was no way I could get the Google sheet to show me the same value as I saw in Excel for the same formula. I had to conclude I couldn't trust Google sheets. If the problem is that I shouldn't have opened an Excel file in Google sheets and instead should have created the file from scratch in Google sheets... that doesn't work for me as an explanation because math is math and the same formula has the same one answer.

It was a simple ISERROR formula, just if, then, then.
I have wondered if anyone else has had that experience.

It returned a value, but not the right one?

That's odd. There are some differences in functions but ISERROR is a pretty simple one.

I've had some errors after conversion but I have generally been able to fix them.

Yes, it returned a value but not the right one. The scary part was, I was about to hand over a workbook designed for specific calculations based on weights (weights of animals, in order to determine medication dosing). If I had not scrutinized/compared the same file on both programs, my friend would have gone on to use the workbook in Google Sheets, and her calculations would have been very very off, and animals might have suffered.