Undo still available after a Worksheet_Change event???

[ac3100]

I am using a Worksheet_Change event in my "Calendar" worksheet when a user changes the value within a cell.

That value goes to another worksheet:

Worksheets("Daily Weekly Tasks").Cells(zz + 3, 7) = Worksheets("Calendar").Cells(22 + (xx * 9), 4 + (yy * 4))

I was amazed to see that the undo button was active.
It said undo typing "1156" in c47.

I pressed it to perform the undo and I got a 1004 error.

1. I thought undo would not be active since a macro was already performed.

2. Why is the error happening. The code works fine under normal circumstances.

Thanks

Andy