"Replace All "not working correctly

A problem with "Replace All" in Excel.

When I enter the characters to be changed into the "Find" field and the new, desired characters in the "Replace" field, and then click on "Replace All", every single cell in the sheet gets filled with the "Replace" text (unless I have selected a column, in which case it's every cell in the column) , regardless of what the cell originally contained. I have over 100 instances to change—how can I do them all at once, not one by one?

I did manage to change all the desired cells in one column and nowhere else when I used auto-filter to select the incorrect cells in that column, selected that column, and used "Replace All". All the incorrect cells were filled with the replacement text and nothing else was modified. But that was a special case; I can't always select just the cells in which the wrong text appears.

How can I get "Replace All" to do the replacement ONLY where the "Find" instance occurs?

(This was registered as case 1038589406)