I am worried along with Peter Todd, Mike Hearn, etc.
There is scenarios were it wouldn't matter what the miners, Ghash, etc do.
It is 100% possible...and if you don't believe it then you should consult for btc core developers.
The problem isn't the double spend...you get 51%, hard fork, and an entity could change the rules for miners and how they are confirming transactions. Please see below.
Check out this great interview with Peter Todd who goes through the process of a hard farking.
https://www.youtube.com/watch?v=lX0imwGHK0AI don't think it would be any 1 pool that would do this for other than monetary purposes...but just that it is possible is concerning and a weakness in the scalability of btc.
I still love you bitcoin :-)