Miguel Ferrer's death (he had cancer) during NCISLA really hit me hard. His character was one of my favorites. I remember thinking in his last few episodes that he must have been struggling with a health issue, but I had no idea until after he passed. He really worked as long as he could, and I'm glad the show had a little bit of time to write in an explanation for the death of his character instead of him just disappearing with no acknowledgement at all.

Separately, I didn't know Cliff Simon (who played Ba'al in Stargate SG-1) had passed away until a few years after his death. (The series had already concluded by then.) But it was still a shock to find out because I was just looking up various actors from the series to see what they were doing more recently