DeNiro as the Irishman. Marty, we know that you love Bobby and have for 50 years, but cast someone believable as gangster muscle.
Besides Sophia Coppola, nepotism also got Talia Shire the role as Connie, also a weak point.
How can people talk about miscasting in Batman without mentioning Silverstone and O'Donnell?
Cary Elwes as a southerner in both Days of Thunder and Twister. Maybe he was doing a bit in tribute to Costner's Robin Hood.
Broderick was the perfect Ferris Bueller, at least as far as Hughes films go. Hughes doesn't do face-jock, athletic, prom king or queen heroes, he wants you to root for the little, different guy to win. Farmer Ted / Samantha, Annie / Duckie, Keith / Watts, Uncle Buck, Del Griffith, Brian Johnson, Gary and Wyatt.