The "MDI bug" is one of those Clarion-related issues that's been around for a long time, but which is somewhat lacking in hard documentation. The usual manifestation is multiple minimize, restore and close buttons on the MDI child window.
You've probably heard the story of how SoftVelocity spent a lot of time tracking down the bug, finally sending some sample code to Microsoft. And Microsoft verified the problem, said it would not be fixed, and changed the documentation,.
I was reminded of this by a post Gustavo Pinsard made in the sv.clarion.clarion8 newsgroup. In response, Diego Borogovitch said:
The problem is not with MDI but with threaded MDI. MS does not support windows to be open in multi threaded MDI because the part of code in the OS that manage the MDI is not thread safe.
Lee White pointed out that problem manifests itself when you maximize child windows, and several other devs agreed that as long as you get rid of the maximize option you should be good to go. Resizing windows is fine.
Gustavo noted that removing the MDI Tab feature seems to have fixed his issue.
So, what's the moral of this story? The MDI bug is there and most likely will be around for as long as MDI is around. Just don't let your users maximize MDI child windows.