After starting to use a minimize-to-tray app, I was getting a number of invisible windows with the Plus! logo. After a bit of investigation, I found that they were subclass windows I used in my scripts.
I've also noticed (since a while ago, but I've only recently worked out why it happens) that when Messenger freezes, all the subclass windows appear as small blocks (just showing a portion of the title bar since the windows are 0px by 0px) in the middle of the screen.
Just wondering: is there a way of making them "more hidden"?