Many people are already aware of the well-publicised issue with upgrading to 8.1.6 due to historical corruption in the data dictionary. (If you are not aware of such a nasty problem, check out Note:96117.1 on metalink. Basically your database may refuse to open after upgrade... You DID take that pre-upgrade backup didn't you?!)
A simple one which may affect you is the V$BUFFER_POOL_STATISTICS view. I mention this mainly because its existence in versions earlier than 8.1.6 typically are representative of a DBA acting in good faith to try manage the buffer pools. This is because you had to create it manually using the catperf.sql script.
However in 8.1.6, V$BUFFER_POOL_STATISTICS is defined in kqfv.h and is thus a "genuine" fixed view. If you had manually created the view, then after upgrade, you will have the peculiar anomaly of the view being INVALID when viewed in DBA_OBJECTS, but you can still freely select from it. Also any attempt to drop the invalid view will result in an error (because you are trying to drop the new fixed view).
You thus may want to get rid of it BEFORE you upgrade.