There's a bug in sys.identity_columns: it ignores requests for read uncommitted. This means if someone's rebuilding a large index on an identity column, queries get blocked.

To reproduce it, create a table with a clustered index on an identity column, and rebuild it:

CREATE TABLE dbo.Test (Id INT IDENTITY(1,1), CONSTRAINT PKId PRIMARY KEY CLUSTERED (Id));
GO
BEGIN TRAN
ALTER INDEX PKId ON dbo.Test REBUILD WITH (ONLINE = OFF);
GO

Then in another window, try to query sys.identity_columns with dirty reads:

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
SELECT * FROM sys.identity_columns WITH (NOLOCK);

(I know, those two isolation level hints are redundant, just being clear that nothing you ask for will get you past the blocking on sys.identity_columns.) Reproduced on SQL Server 2017 CU 17 (14.0.3238.1.)