I think the point of that scene is to show that, under the severe pressure Karen was feeling because of Henry's arrest, she was getting paranoid.
I agree. I'd add that the entire final 45 minutes of the film is dedicated to the paranoia involved in living the drug culture. Helicopters, lucky hats, bimbos, etc.
"I got news for you. If it wasn't for the toilet, there would be no books." --- George Costanza.
