Is the Hollywood portrayal of the effects of ingesting HCN correct?
Is there anything realistic about that scene, or is it just another movie gaffe?


I have never heard about anything similar caused by $\ce{HCN}$ or $\ce{KCN}$.

However,


bleeding of the gums, 
loss of teeth, and 
osteonecrosis of the jaws (= the bone tissue literally rots away)


are known symptoms of certain intoxications:


Bisphosphonate-associated osteonecrosis of the jaw was observed in the treatment of patients with cancer
Radium jaw was observed in painters (Radium girls) that licked their paintbrushes while applying radioluminescent paint to dials
Phossy jaw was observed in (mostly female) workers in the matchstick industry of the late nineteenth century that worked with white phosphorus. 

Back in the days, the lives of the patients often could only be saved by removing the jawbones surgically. (If you feel for it, look for some photographs online.) Since one of the tags is chemistry-in-fiction, you might be interested that this was partly covered in the episode Become Man of the BBC series Ripper Street.