What happens If a table changes during a query/procedure execution

Let there are two transactions that were created with read commited
isolation level. In the first one we're executing a SELECT query:
SELECT * FROM t UNION ALL SELECT * FROM t;

In the second transaction we're modifying the same table:
INSERT INTO t DEFAULT VALUES;
COMMIT;

Is it possible that the last UNION part in the first query will retrieve
not the same rows as the first one?
Another scenario is where we're executing two SELECT queries in a stored
procedure:
BEGIN
...
SELECT * FROM t;
SELECT * FROM t;
END;

Is it possible to get different results in the second query? Does SQL
standard define the behaviour in such cases?