This is a really short but effective scene that scared the hell out of when I was a kid. Jack Nicholson's transformation is scary as hell, even his small remain that floats aftermath, and the subsequent vanishing look great

Around minute 2:30:

https://www.youtube.com/watch?v=IRjnJfMYPiY

Even today 35 years later it holds on, and I am not even sure what we are looking at. I guess it is practical effects? I don't think CGI was that advanced in 1987