Answer
Only 50 rasters of the ESRI GRID format can be open at one time in a Visual Basic session in pre-ArcGIS 9.0 versions.
As a workaround for pre-ArcGIS 9.0 versions, if there are temporary grids, save some of them to disk and set the temporary grid object to nothing. Also, close some of the open grids and open them only when it is necessary to access them.
At ArcGIS 9.0 the above limits have been removed.