One of the bugbears since making the upgrade to OS X 10.10 Yosemite is how it handles full screen switching. In the past, you could click the green icon found in the top left of each window to make the active window large but not enable full screen mode (this was done from the top right corner if the applications supported this).
However this user experience has been changed in Yosemite; now when pressing the green button found in the top left corner of an application window, this enables full screen mode (again, only for those apps that support full screen).
This is something that was slowly driving me made as out of rabbit I would click the green button to make it larger but not full screen - OS X has worked this way for years until OS X 10.10 was released.
The good news!
There is a way to avoid this! Before clicking the green button, hold down 'Option / Alt' on your keyboard and then click the green button. You will notice the icon change to the + symbol we are used to seeing in previous OS X versions meaning the window will go larger (as expected) but not full screen..
I hope this little trick helps as much as it did me.