The Sorry About The Scabies Cake is a cake that SpongeBob purchases for Patrick's birthday, from the bakery, in the episode "Overbooked." He asks the baker if she could change it to say "Happy Birthday," but she refuses, and SpongeBob just purchases it anyway. It has light yellow, blue, green, pink and light purple icing.
Scabies is a skin disease in humans where mites burrow into the skin and cause a rash that is similar to mange in animals with fur. In this case, scabies is an STD to those in Bikini Bottom, like it would be in the human world.
According to Casey Alexander's Twitter feed, the scabies joke was based on his experience with it at the age of 10.
"I'm allowed to make the scabies joke 'cause it comes from personal experience. My 10 year old self was delighted w/ that one." -