Nightmare Fuel: Snoopy is abused and starved, becomes evil, and then dies getting pulled into a frozen lake (alongside the other sled dogs and the cruel master), screaming and clawing for his life. Granted, the special establishes from the start that it's All Just a Dream and none of it is really happening, and there is some comfort in the happy ending with Snoopy eating ice cream and being allowed to sleep in Charlie Brown's bed for the night, but still... one could argue that this was inappropriate for the Peanuts specials' target audience.
Especially the part where Snoopy growls at that husky.
Tear Jerker: For anyone who loves Snoopy, the whole special. We're forced to watch the beloved beagle subjected to horrific abuse, break down in despair, regress to a feral state, and ultimately drown in frozen water. All because he enjoyed the comforts of home too much.