The Fill command incorrectly fills a sink if it is one cell off the edge of a DEM and the pour point is along the edge. The Fill command works correctly for any other sinks that might exist in a DEM.
This is a known issue with ArcGIS 8.1, ArcInfo 7, and ArcView 3.2.
The problem arises when there are sinks near (only one cell off) the edge of a DEM and the pour point is along the edge. Instead of using the elevation value of the pour point, the Fill command fills the sink (and possibly some other adjacent cells) using an elevation value higher than the pour point.