When Zack falls asleep, the string to work the light is attached to the bed on his right side. When he wakes up, the string is attached to the bed on his left side.
Lisa's earrings change from red and white hearts to solid red circles.
Screech gives Zack a frog's foot for a good luck charm. Zack flicks it away to get rid of it, and it lands on the hospital bed between his knees. When Zack gets up later, the frog's foot is missing.
After the surgery, when Zack's entire leg is inexplicably put into a cast, the cast is applied OVER his pajama pant leg. That would never happen since casts are usually worn for at least a month and are not applied over clothing.
At no point during his hospital stay (including the day of his surgery) is Zack seen wearing a hospital gown which the hospital should have required him to wear. He would not have been allowed to wear his own pajamas.
Following surgery, Zack's leg is put in a full leg cast (as if the leg was broken) despite the fact that it was a relatively minor operation on his knee.
Both Zack and Lisa seem overly concerned about Zack's relatively minor knee surgery operation. The knee surgery would not have put Zack's life in danger.
When Screech accidentally moves Zack's adjustable hospital bed, the bed moves much faster than a real hospital bed would.