If I require_once both the Flow and OOUIPlayground extensions, I get
Fatal error: Call to undefined method Flow\Container::share() in /home/spage/projects/core/extensions/Flow/container.php on line 21
I think this happens because Flow invokes Pimple 1.x's share() method, but OOUIPlayground loads a later version of pimple that "wins". See Undefined Function share() #135 issue. I wasn't able to fix by changing the order of the require_once.