Hard to say much without a few more specifics, but ..

If the behaviour is different then something in the environment must be different. If you are not changing anything in your workbook (including the active sheet or cell) then it must be in your VBA environment - do you have any global variables? or do you create objects but not delete them afterwards?

It would help at least to know what statement is giving the error. What is the subscript? What is being subscripted? How are any variables defined?