1. After deleting a column, you are unable to recreate the column. The system is returning an error that the column already exists. However you don't see the column when you look at the sys_dictionary table. 

2. After deleting a column, you notice that the column is still showing in some parts of the platform.

All

The cause is usually a failed online alter for the deletion process. This usually occurs when deleting a column from a very large table, like task or cmdb. You won't see an error in the UI, so you will think everything was deleted properly, however you will find remnants of the column throughout the different tables that make up a column record.

If you suspect a column didn't delete correctly, check the following tables for records relating to the column you deleted:

-- sys_dictionary

-- sys_documentation

-- sys_storage_alias

-- sys_metadata

You will need to open a case with customer support to have the column cleaned up and completely removed from your instance.