In case authentication takes some time (e.g. kerberos server being unavailable and fallback to local unix account has not yet tried) the login process looks as follows:
1. user selects the account
2. password field appears, prompting for password
3. user enters the password
4. password field is emptied, again prompting for password.
At this time it is possible that the user will try to make several more attempts before the response from the low level received allowing the login. GDM was disabling the UI while waiting for authentication response to signal that the machine has understood the request and is doing something. In this case it is not clear whether anything has actually happened.
The same can be seen during local auth only - upon pressing Enter the password field is cleared and the blinking cursor appears for a second, this also will be a problem with encrypted home directories on slower machines since mounting the encrypted directory takes time.
Do you know what greeter and version you are using? This should be fixed in both the GTK greeter and Unity Greeter (the password entry goes insensitive after entry).