Submitted by: @hvlad

Run Firebird 2.1 or 2.5, CS or SC

1)
1st isql session :
...\firebird\bin>isql
Use CONNECT or CREATE DATABASE to specify a database
SQL> create database 't.fdb';
SQL>

2)
2nd isql session :
...\firebird\bin>isql t.fdb
Database: t.fdb
SQL>

3)
1st isql session :
SQL> create table tmpdat(id int not null primary key);
SQL>

4)
2nd isql session :
SQL> drop table tmpdat;
Statement failed, SQLSTATE = XX001
database file appears corrupt (...\FIREBIRD\BIN\T.FDB)
-wrong page type
-page 0 is of wrong type (expected 6, found 1)
SQL>

5) Validation found no errors

Firebird before v2.1 also have this issue but it have different consequences:

1st isql session :
...\firebird\bin>isql
Use CONNECT or CREATE DATABASE to specify a database
SQL> create database 't.fdb';
SQL>

2)
2nd isql session :
...\firebird\bin>isql t.fdb
Database: t.fdb
SQL>

3)
1st isql session :
SQL> create table tmpdat(id int not null primary key);
SQL> insert into tmpdat values (1);
SQL> commit;
SQL>

4)
2nd isql session :
SQL> drop table tmpdat;
SQL> exit;

5)
1st isql session :
SQL> exit;

6) Run validation
...\firebird\bin>gfix -v -fu T.FDB
Summary of validation errors

    Number of record level errors   : 1

firebird.log contains
Database: ...\FIREBIRD\BIN\T.FDB
Record 0 is wrong length in table (128)

Commits: 272adc2 53dd0b1 9ddc0ef