I just tried it with manual calculation permanently on, and it still
happens.

After posting this, I discovered that if you DON'T interrupt it, the
over-simplified statement "Sheets("sheet1").Copy" actually copies the
sheet into a newly-created Book2. This hints at where the phantom
Book2 may come from.

But the same thing does happen when you interrupt copying within the
workbook, e.g.,

Sheets("sheet1").Copy befo=Worksheets(1)

It's almost as if interrupting this statement makes processing stop
after the "Copy" and before the "befo=" and it aborts in such a way
as to sort of half-create Book2.