Can a transformed Saiyan lower his ki to base-human level when transformed? In Dragon Ball Resurrection of Freezer, Goku, when transformed in Super Saiyan God Super Saiyan, is almost killed by a laser beam from Sorbet. Now how could this happen? Was his ki so low even when he was a SSGSS? Later in the Dragon Ball Super series this seems to be corrected because he isn't transformed when he is hit. But then again, I don't know which is the version that counts, supposedly the manga is over the movie and series but between something that happens in the movie and series I don't know.

Then the question of this topic is: can a transformed Saiyan (Super Saiyan, Super Saiyan God, etc.) lower his ki to base-human level when transformed? Was this just a mistake or what?
There are kind of multiple issues wrapped into one here. Firstly, 


  Can a super saiyan their ki to the point that it is human level?


No that is impossible. Why? Because it takes a great amount of energy to undergo and sustain a physical transformation. If you drop your ki below a certain point. You revert to your base form. 


  Goku death by beam? 


Yes that can also happen. It seems almost impossible in theory but remember saiyans are in fact humanoid aliens and as such they can fall vulnerable to being shot in the back if they lower their ki sense. Whis warns Goku of this when he tells him during their training and in the manga that he gets too overconfident easily and lowers his guard and that is his weakness which is the polar opposite of Vegeta who is too on-guard and thus gets slowed down. (I can link the manga panel later). 

The point is. Goku was confident of victory which is why he was able to be shot.