If I create an index, and then use a hint in a query to use that index, the query fails with:
Msg 308, Level 16, State 1, Line 1
Index 'abc' on table 'dbo.abc' (specified in the FROM clause) does not exist.
With tools like DTA, the dropping of indexes is going to become much more common, but common sense of inspecting code for index hints is not necessarily going to get any better, until people get bitten... and even then they will probably react by only fixing the one query.
[Filed on behalf of Greg Linwood.]