Here's a minor bug that could be a RenPy bug. This is on a late 2006 17" iMac 2 GHz Intel Core 2 Duo running Mac OS X Lion 10.7.5.
When I launch in windowed mode, I have two menus on the menu bar: Katawa Shoujo and Window
The Window menu starts with two entries, "Minimize" and "Katawa Shoujo" (which has a checkmark). This is apparently for VNs that have multiple windows.
Every time I change to fullscreen mode and back, another "Katawa Shoujo" is added to the Window menu. Same thing if I resize the window manually. If I minimize the window, I get a new "Katawa Shoujo" when I un-minimize it, but the extra "Katawa Shoujo" is only added the first time I do this.
If I select any "Katawa Shoujo" entry except the last one, I get a blank window the same size as the real game window, and the game crashes in a few seconds. If I select the last "Katawa Shoujo" entry, that is apparently the real game window and it's fine. If I minimize the game window, the last entry is tagged with a diamond to show I can un-minimize it.
It looks like resizing and the first minimize creates, or thinks it creates, an additional new window.
PS: This is a RenPy bug; trying version 6.14.1 and using Renpy's tutorial game, it does exactly the same thing.
PPS: For people testing this:
A full list of changes to Ren'Py can be found at: http://www.renpy.org/doc/html/changelog.html
(KS is going from 6.10.2e to 18.104.22.1680)
A list of changes that may require you to update your game can be found at: http://www.renpy.org/doc/html/incompatible.html
For example, in 6.15, the Macintosh version of Ren'Py now requires a 64-bit capable processor, and Mac OS X 10.6 or newer.