I've narrowed the most likely cause of the problem to the actual code that fires in the event. It creates a vCal file in memory and streams it to the client. The odd part of it is that it worked up until I made some unrelated changes to the user control in which the Repater sits. I'll probably just hack it and make the ItemCommand open a page that handles creating the vCal stand alone.

Protected Sub RepeaterShowGateway_ItemCommand(ByVal source As Object, ByVal e As System.Web.UI.WebControls.RepeaterCommandEventArgs) Handles RepeaterShowGateway.ItemCommand

        Select Case e.CommandName
            Case "CreateCalendarItem"
                Exit Sub
                'http://www.codeproject.com/KB/aspnet/vcalendarfiletodownload.aspx
                'PARAMETERS
                Dim LabelNameLong As Label = CType(e.Item.FindControl("LabelNameLong"), Label)
                Dim LabelBranchNameShort As Label = CType(e.Item.FindControl("LabelBranchNameShort"), Label)
                Dim LabelShowNumber As Label = CType(e.Item.FindControl("LabelShowNumber"), Label)

                Dim vcal_LabelTFCMoveIn As Label = CType(e.Item.FindControl("vcal_LabelTFCMoveIn"), Label)
                Dim beginDate As Date = vcal_LabelTFCMoveIn.Text

                Dim vcal_LabelTFCClear As Label = CType(e.Item.FindControl("vcal_LabelTFCClear"), Label)
                Dim endDate As Date = vcal_LabelTFCClear.Text

                'INITIALIZATION

                Dim mStream As New MemoryStream()
                Dim writer As New StreamWriter(mStream)
                writer.AutoFlush = True
                'HEADER

                writer.WriteLine("BEGIN:VCALENDAR")
                writer.WriteLine("PRODID:-//Flo Inc.//FloSoft//EN")
                writer.WriteLine("BEGIN:VEVENT")
                'BODY

                writer.WriteLine("DTSTART;VALUE=DATE:" & _
                         beginDate.ToUniversalTime.ToString("yyyyMMdd"))
                writer.WriteLine("DTEND;VALUE=DATE:" & _
                        endDate.ToUniversalTime.ToString("yyyyMMdd"))
                writer.WriteLine("LOCATION:")
                'Add Links here to Booking Report, QF, Sharepoint Folder and Show Site Dashboard
                writer.WriteLine("DESCRIPTION:" & "Message Body Here")
                writer.WriteLine("SUMMARY:" & LabelNameLong.Text & " " & "(" & LabelBranchNameShort.Text & "-" & LabelShowNumber.Text & ")")
                'FOOTER

                writer.WriteLine("PRIORITY:3")
                writer.WriteLine("END:VEVENT")
                writer.WriteLine("END:VCALENDAR")
                'MAKE IT DOWNLOADABLE

                Response.Clear() 'clears the current output content from the buffer

                Response.AppendHeader("Content-Disposition", _
                         "attachment; filename=OutlookAppointment.vcs")
                Response.AppendHeader("Content-Length", mStream.Length.ToString())
                Response.ContentType = "application/download"
                Response.BinaryWrite(mStream.ToArray())
                Response.End()
            Case Else
        End Select

    End Sub