Probably the worst I can think of is taking the Google Android playbook open with closed extensions then deprecate the open. You already see this a little bit with things like JavaFx in the Oracle distribution but not the OpenJDK. Sure you can build it yourself, but how long till we see extensions that are in the Oracle distribution that can't be built for the OpenJDK? Eventually it looks like Android where what is open and what people actually use are two different things.