Not sure how this is happening, but it is.  I assume through facebook log in.  Can't see any settings anywhere regarding this?

I have the same problem. But not new in 4.2, 4.1.x also.

coming through facebook?  i'm on 4.1 also

Why through Facebook?

You can set for each of the social media login handlers how display names should be constructed. If you give users the choice to pick a name, but they don't pick one and leave, the account won't have one.

Thanks for the reply.

I had it set like to not use their facebook name as in the image, and assumed they would HAVE to pick a username, but apparently not?  So the only option if using facebook is to use your real name?

2 hours ago, Steve Bullman said:

they would HAVE to pick a username

Yes. But as already mentioned: they can just leave the page, when they are presented with the choice for a display name. Then you end up with this account in the database, that doesn’t have a display name. I think they are cleaned up after a while. 

gotcha, so its basically an unfinished attempt at creating an account.

I've had that too ever since 4.0 upgrade last year and also have real name switched off

have real names on and been getting this as well

 

I thinks it is a bug, because it only appears when user cancel the registration process with Facebook, Google or Twitter.