Continuity mistake: When Josh just becomes big, he hands his baby sister a wallet. She lowers her hands beneath the border of the playpen. The immediate angle, barely a frame later, shows her hands above it. (00:14:45)
Continuity mistake: When Josh and Billy are in the equipment room note the orange and blue pads on the middle shelf. The blue pads change, their white strap goes being tucked up to hanging down then they disappear completely and reappear between shots. (00:16:35)
Continuity mistake: Susan's hands change position on Josh's neck as she says goodbye at the end. (02:04:10)
Continuity mistake: John Heard's racquet jumps from one hand to the other when he and Tom Hanks fight on the tennis court. (01:17:20)
Answer: According to imdb: Though some believe it to be Colossal (Cave) Adventure or an early Sierra Game, no known game up to 1988 accepts the commands "melt ice wizard" or "throw thermal pod" (as used in the movie), therefore one can assume the game screen was simply created for the movie.
Carl Missouri