When previewing a feature class in ArcCatalog that is stored in Oracle Spatial format, the following error will appear in $SDEHOME/sde.errlog,
"load_buffer error -2028"
There are not enough vertices to create a valid shape. For example, lines require at least two vertices, and polygons require at least four, with the first and the last being in the same place.
This is a data problem. Either there are not enough vertices in the feature, or ArcSDE is rejecting some parts of the feature as invalid, leaving insufficient vertices to complete the feature.