The appearance of Amy's (Mayim Bialik) dad in Young Sheldon is not a plot hole, unlike George's cameo in The Big Bang Theory. 

After getting his first pimple, Sheldon becomes obsessed with the idea of hitting puberty and developing a fear of growing old. George (Lance Barber) and Mary's (Zoe Perry) current predicament aggravates his trepidation. In an effort to interject some humor into an otherwise darker Young Sheldon episode, CBS brings in the magician duo to personify Sheldon's pimple, with Teller playing the voice of Pus while Penn brings Acne to life.

This is supported by the fact that Lance Barber, who plays George in the CBS prequel, had previously appeared in The Big Bang Theory as Leonard's bully, Jimmy Speckerman — something that is deemed a plot hole. That being said, Teller's Young Sheldon cameo doesn't actually create any inconsistency because the character he plays is imaginary. Pus and Acne only appear in Sheldon's dreams and don't have any real-life impact on his life in Young Sheldon.