I have MacHg an a certain Mac OS X Space. Now, I wanted to push some change to a remote repository which - for some reasons - resulted in an error. However, I was at a different Space when MacHg reported that error. So my MacHg icon in the dock started jumping. I clicked it (or switched to the Space directly - I don't remember exactly) but there was no error message visible.
I then by accident found out that the error message dialog is only visible during the animation that happens when switching to the Space in which MacHg is contained in (using Ctrl + Arrow Key). As long as this animation is on going the dialog is visible. When the animation is completed the error dialog is gone again. I can reproduce this behavior every time I switch to the MacHg Space.
(By switching a lot I found out that the error message reads: "Error During Push")