When the T-800 blasts the T-1000's hook off while he's escaping from the asylum in a car, the hook stays in the car since it was blasted off. Then you see John touch it and fling it into the road. The T-1000 has now "sampled" John and could have changed into his form at any time, but for some reason, he didn't cycle through him.
I believe that to "SAMPLE" John, it has to be an active intentional "SAMPLING" of the DNA by the T-1000. When John throws the hook back, the hook is INACTIVE and not directly connected to the T-1000. So in that sense, you are technically WRONG.
Hope that clears things up for ya!
--Xo
Why is this a slipup? Let's assume that the T-1000 did "sample" John and COULD now impersonate him. Just because he/it CHOSE not to doesn't make it a slipup. And if you're saying it's a slipup because he/it didn't "become" John during his/its death scene, then it's still not a slipup--there were several people/things that the T-1000 "sampled" but didn't cycle through in the death throws (the floor, the dog, the T-800, John's friend who he touched in the arcade, etc.)