Well all minecraft launchers do exactly that because authenticating to minecraft.net requires a plain text password, here is the code from technics own launcher using the plain text password exactly as feed the beast does:
public static String[] doLogin(String user, String pass, JProgressBar progress) throws BadLoginException, MCNetworkException, OutdatedMCLauncherException, UnsupportedEncodingException, MinecraftUserNotPremiumException, AccountMigratedException {
String parameters = "user=" + URLEncoder.encode(user, "UTF-8") + "&password=" + URLEncoder.encode(pass, "UTF-8") + "&version=" + 13;
String result = PlatformUtils.excutePost("https://login.minecraft.net/", parameters, progress);
As you can see the login string is created using the plain text password which is then sent to mojang to see if the login is valid, the only difference here is the FTB Launcher prints out everything it does to the console so if the login failed (because minecraft.net was down) then the password would be visible in the error, it was clearly a mistake but was quickly rectified as a updated launcher was pushed and all the pastebin's containing the password were taken down.