Other mistake: When Rocky is in the funhouse looking at himself, he appears to look relatively "normal". But it's very obvious that it's got nothing to do with the mirror, and more to do with the removal of a lot of makeup. For example, Rocky usually has almost no nose, but in the mirror he has Eric Stoltz's very well developed nose.
Deliberate mistake: The mirror that made Rocky's face look normal should've made his mom's face look different, but her face stayed the same.
Continuity mistake: At the end of the movie a big man is placing flowers at Rocky's grave. From the back of the tombstone he is laying the flowers on their sides. In the next shot, from the front of the stone, they are standing up, leaning against the stone.
Revealing mistake: At Rocky's graduation, when Dozer hugs him, you can see the make up wrinkle right below his eye.
Answer: His official cause of death was listed as "sudden arrhythmic death syndrome," which is basically just when someone dies unexpectedly, usually during sleep. It's widely believed that it was caused by his disorder, as his disorder often causes early death, but to my knowledge it was never definitively proven.
TedStixon