I don't know if it's OSX itself or one of the common GUI engines implementations on OSX, but I notice a lot of apps (including Firefox when you try to save a file which would overwrite another file) will have popup modals with multiple buttons and there's no way to click the non-default button with just keyboard.

For example, in Firefox, trying to save a file when a file by that name already exists, you get a "Cancel" or "Replace" prompt and you can "Cancel" by pressing return, but if you want to "Replace", you apparently HAVE to use mouse or trackpad!?!