At login screen, entering a username that contains a space, or a single space, will cause the the following "password" prompt to show typed characters in clear text, while users would expect these to be hidden by dots.
In fact, this "password" field is actually a second "login" field, but the label shown is incorrect, tricking the user into thinking that they are entering the password.
I suggest that, when an incorrect login name is entered, LXDM either accepts it silently, or it shows a warning and a keyboard delay so users won't type their passwords in a non-hidden field by mistake. At least, the label shown should be correct.
This is described also in http://forums.linuxmint.com/viewtopic.php?f=175&t=100784, and at least affects other distros like Zentyal distributions 2.2 and 3.0 (reproduced in both, which correspond to lxdm 0.2.0 and 0.4.1).