• Cory Monteith, died of an overdose at 31 while the show was airing.
• Mark Salling, arrested for children porn possession, multiple sexual assault allegations, committed suicide at 35.
• Naya Rivera, died from an accident in a lake at 33.

Those are the big ones.

A bunch of the cast died young, overdosed, etc like they were cursed iirc. I think maybe one murdered their parents or something? That might have been another show.