<% Session("StoredArray")(3) = "new value" %>

This is because the Session object is implemented as a collection. The array element StoredArray(3) does not receive the new value. Instead, the value is indexed into the collection, overwriting any information stored at that location.

It is strongly recommended that if you store an array in the Session object, you retrieve a copy of the array before retrieving or changing any of the elements of the array. When you are done with the array, you should store the array in the Session object again so that any changes you made are saved. This is demonstrated in the following example:

---file1.asp---
<%
'Creating and initializing the array
Dim MyArray()
Redim MyArray(5)
MyArray(0) = "hello"
MyArray(1) = "some other string"

'Storing the array in the Session object.
Session("StoredArray") = MyArray

Response.Redirect("file2.asp")
%>

---file2.asp---
<%
'Retrieving the array from the Session Object
'and modifying its second element.
LocalArray = Session("StoredArray")
LocalArray(1) = " there"

'Printing out the string "hello there."
Response.Write(LocalArray(0)&LocalArray(1))

'Re-storing the array in the Session object.
'This overwrites the values in StoredArray with the new values.
Session("StoredArray") = LocalArray
%>

（出处：chinaasp）