If database was shut down by the DBA, existing client connections are received appropriate error and should close its connection. Since v2.5 network server don't wait for client to close connection, instead server waits for any packet from client, responded with shutdown error and closed connection immediately. In some cases client could wrongly detect it as error and put following messages into firebird.log :
INET/inet_error: read errno = 10054
REMOTE INTERFACE/gds__detach: Unsuccesful detach from database.
Uncommitted work may have been lost