Tested using the mysqli code, but most likely happening with mysql too:

In master/slave mode, when the ezdb object is created, it tries to connect to writedb and readdb. It calls thus twice the connect() method, which in turns calls $this->setError(); (line 143 in 450beta1).
The problem is that on teh 2nd connect() call, $this->IsConnected equals true, but $this->DBConnection is false. Hence a php warning is generated by calls to mysqli_connect_error and mysqli_error and mysqli_errno