Chrome Version: tested with 57.0.2987.21, 58.0.3014.0 
OS: Windows

1. Launch the MD sign-in dialog. (e.g., from a non-signed-in profile, choose "Sign in to Chrome" from the profile menu)
2. Click "More options", then click a menu item (e.g. "Find my account")

As the page transitions to the next step, the top-left paper-icon-button, which is showing the back arrow, very briefly flickers to an X (ie, close), then back to the back arrow.

Maybe the X shouldn't become a back arrow when opening the "More options" menu in the first place.