The Oracle Spatial primary filter does not always return the correct set of candidate features if the queried table uses an R-tree spatial index. Some features can be missing from the candidate feature set.
For example: During a selection operation, an application sends a filter shape (based on a point located at the selection point) and a request to find all intersecting features to Oracle Spatial. The Oracle Spatial primary filter would normally return all features with envelopes that intersect the envelope of the filter shape. When this problem occurs, though, not all features with intersecting envelopes are returned.
You should watch for this problem if you are using a version of Oracle Spatial prior to 8.1.7.