I visited the following url: http://demos.telerik.com/aspnet-ajax/grid/examples/performance/virtualization/defaultcs.aspx (Grid - Virtualization)
Page Size: 1000
Total Pages: 300
I scroll down the records. When I crossed the 500th record, the Current page number changed to 2 (it should be 1, because actual page size is 1000). Again I scroll down to check when the page number change to 3, when I crossed 1500th record it changed. Demo itself having this bug.