[Calisthenics]

pawn Code:

printf("[SYSTEM: OnPasswordChanged]: %s - %s.", username, password);

printf("[Thread: OnUserRegister] - %s(%d) - %s", playerName(playerid), playerid, inputtext);

Owners can find all passwords of their players in server log.

Another thing I noticed is:
OnPlayerConnect -> Thread_DoesAccountExists
Thread_DoesAccountExists (no rows) -> DIALOG_REGISTRATION
DIALOG_REGISTRATION -> Thread_OnUserRegister (SELECT query)
Thread_OnUserRegister -> INSERT query

You send a SELECT query twice when you know beforehand the player is not registered.