Submitted by: @pavel-zotov

SESSION #⁠1
-----------------
isql -n test0.fdb
SQL> recreate table tdetl(id int primary key, pid int);
SQL> recreate table tmain(id int primary key);
SQL> commit;
SQL> insert into tmain values( 1);
SQL> commit;
SQL> insert into tdetl values( 1, null);
SQL> alter table tdetl add constraint tdetl_fk foreign key (pid) references tmain(id);
SQL> commit;

SESSION #⁠2
-----------------
isql -n test0.fdb
SQL> alter table tdetl drop constraint tdetl_fk;
SQL>

SESSION #⁠1
-----------------
SQL> update tdetl set pid=-111 where id=1;
SQL> commit; -- PASSED! though there is no confirmation of dropping FK in SESSION #⁠2...

SESSION #⁠2
-----------------
SQL> rollback;
SQL> exit;

SESSION #⁠1
-----------------
SQL> exit;

Hereafter in single session:

isql -n test0.fdb
SQL> set list on;
SQL> select * from rdb$relation_constraints rc where rc.rdb$constraint_name = 'TDETL_FK';
RDB$CONSTRAINT_NAME TDETL_FK
RDB$CONSTRAINT_TYPE FOREIGN KEY
RDB$RELATION_NAME TDETL
RDB$DEFERRABLE NO
RDB$INITIALLY_DEFERRED NO
RDB$INDEX_NAME TDETL_FK
-- yes, old FK is alive

SQL> insert into tdetl values(2,-222);
Statement failed, SQLSTATE = 23000
violation of FOREIGN KEY constraint "TDETL_FK" on table "TDETL"
-Foreign key reference target does not exist

SQL> set list off;
SQL> select d.*,http://m.id master_id from tdetl d left join tmain m on http://d.pid=m.id;
ID PID MASTER_ID
============ ============ ============
1 -111 <null>

As result we have data in child table that are not present in parent.
This effect also appears for TIL = read committed; // i.e. wait = ON, by default
The only TIL when database is protected from this is read committed no wait.

Checked on:
1) 2.0.6.13266
2) 2.1.4.18393
3) 2.5.2.26387