Msg 208, Level 16, State 0, Line 1
Invalid object name '#foo'.

Msg 3701, Level 11, State 5, Line 1
Cannot drop the table '#foo', because it does not exist in the system catalog.

IF object_id('tempdb..#foo') IS NOT NULL

CREATE TABLE #foo (bar int)
GO
CREATE TABLE ##bar (foo int)
GO
SELECT object_id('tempdb..#foo'), object_id('tempdb..##bar')
GO

SELECT object_id('tempdb..#foo'), object_id('tempdb..##bar')