Steps to reproduce:

1) Enter "Some data" in Sheet2.A1
   Enter "Xyz 999" in Sheet2.D8
   Enter "Abc 999" in Sheet1.A1
2) Select cell Sheet1.A1, then select the text “999”, either in the cell, or in
the formula bar.
3) Copy the selected text to the clipboard (Ctrl+C).
4) Open the “Find & Replace” dialog (Ctrl+F); don’t click outside of cell A1.
5) Paste the copied text “999” in the “Search for” box.
6) Tick the “Search all sheets” option.
7) Click “Find”.
The search should find cell B8 on Sheet2.

**Note that the contents of cell Sheet2.A1 have been replaced with the contents
of Sheet1.A1.**

The contents of the starting cell are copied to the same address on whatever
sheet the search first lands on. No spurious copy occurs on repeated searches.

The copy (step 3) is necessary to trigger the problem; the paste (step 5) is
not. E.g., in step 5, type the string “Gamma” in the “Search for” box—the
unwanted copy will occur on Sheet3.A1.

In my testing, this occurs in 2.4.2, 3.1.1, and 3.2rc5.