Also in Season 9, Cliff from Season 2 of Top Chef makes an appearance when they visit BLT Steak.
Speaking of Top Chef, during the charity dinner of Episode 12 in Season 9, it's FABIO!!!
Apparently, before getting on Hell's Kitchen, Clemenza ended up cooking for Robert De Niro, which resulted in him having cameo appearances in films like A Bronx Tale and The Great Expectations, the latter of which involved him stabbing Robert's character.
What Could Have Been: Gloria was originally supposed to stay as the red team's sous chef in Seasons 6 and 7, but either quit or was ditched at the last minute, resulting in Heather acting as a temporary replacement during Season 6, and Andi coming on-board as the permanent replacement for Season 7. It's not known exactly why this happened; some say the producers ditched Gloria because they didn't like her somewhat mopey demeanor and wanted someone younger and more attractive, while others say Gloria quit because she wasn't comfortable with the attention she got from appearing on the show.