If you are using SQL table hint NOLOCK, you would expect that this hint would always be honoured.
However when reading data from a table this will always cause a schema stability lock (Sch-s); even when the NOLOCK hint is used. The latter schema stability lock will block any data definition language statements (i.e. alter table) that which to change the schema (metadata) of the table that is being read.
Ideally the NOLOCK hint should be renamed to reflect this; perhaps SchemaLockOnly.