Gundo wreaks havoc with WinEnter WinLeave events, or window size.
I'd stuck a couple lines in my vimrc to automatically resize a vertically split window when switching to it. That way, I could have a reasonable font size on my crappy monitor and still have my .cpp's and .h's side by side.
But I've since had to modify my mappings to mostly disable this behavior while a Gundo window is open. Even then, Gundo usually resizes the left-most window to as wide as possible, even if the pertinent window is to the right. This might still be my own script doing a couple things right before the "Gundo" buffer has actually loaded.