I received this error message today while trying to set up a full-text index on SQL Server 2016. I have decided this is the best error message ever.

SQL72014: .Net SqlClient Data Provider: Msg 7653, Level 16, State 2, Line 1 'some_index' 
is not a valid index to enforce a full-text search key. A full-text search key must 
be a unique, non-nullable, single-column index which is not offline, is not defined 
on a non-deterministic or imprecise nonpersisted computed column, does not have 
a filter, and has maximum size of 900 bytes. Choose another index for the full-text 
key.

Well, I guess I know what I did wrong. ☺