The Danza: Ryan Matthews is played by Ryan Eggold.
Riley is played by Riley Smith.
Dawson Casting: In the extreme, to the point where this trope is often called "90210 Syndrome" outside of this wiki, since the extremeness makes Dawson's Creek look like they cast age-appropriately.
Though there were some aversions (Brian Austin Green, Tori Spelling, Jennie Garth, and a few others), the straight examples were very extreme (Most infamous was Gabrielle Carteris who was twenty-nine when she started on the show).
Dye Hard: Silver is a natural blonde as shown in the parent shown, but is a brunette for the entirety of the series save for when she briefly dyed it red.
Executive Meddling: Season two drops Ethan and focuses a lot more on the new characters rather than the old cast. That's cleverly done for Kelly's mother's funeral as it's not shown and Silver comments that a lot of people that knew her but she didn't know attended.