Consider this table definition:
create table t ( i serial, b bool, c bool,
constraint b_c check ( (b = true and c is not null ) or (b is distinct
from true and c is null) )
constraint b_c check ( (b = true and c is not null ) or (b = false and c
is null) or (b is null and c is null) )
);
Despite the above two constraints, the following rows get into the table:
insert into t (b , c) values (null, true), (null, false);