Seinfeld. I remember when it came out everyone was telling me to watch it but I just did not have a desire because I never cared for Jerry's standup routines and figured it would suck. So I did not start watching them until they got into syndication. Still did not care for Jerry but the others made it great.
Star Trek Enterprise. I think when it first came on it was on a channel that was not on our cable system at the time. After the series was over I think SciFi got them and I watched all of them.
The Sopranos. I started watching it around the 4th season. Did not know what was going on with some of the story lines and characters. So the wife's grandfather bought the first 3 seasons and we watched them. After that we would watch Sopranos together.
Two and a half men - To this day the only ones I have seen were the ones in syndication that are played around 6 or 7 in the evening. Never watched any live/prime time ones.
How about some shows you never watched...
Family Guy - scene a few clips here and there but never a full episode.
American Dad
South Park - Although I did watch the movie for some reason
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