Row in USER_INDEXES table not deleted when constraint is dropped

If I create a unique constraint (using "alter table x add constraint ..."), then Oracle will create a row in the USER_CONSTRAINTS table and another row in the USER_INDEXES table. If I drop the constraint (using "alter table x drop constraint ...") then Oracle deletes both rows most of the time... but sometimes the row in the USER_INDEXES table is not dropped. Can anybody tell me what may be happening? Running 10g, 10.1.0.2.0. Received on Fri Oct 06 2006 - 17:11:20 CDT