Well, Shannow forgive me for bringing it up AGAIN, but a literary example would be wheel of time. I didn't read the last books by the new author because they came out too long after the last book by Jordan, so I'd have to reread the whole series again from the start to know what's going on, and there's NO WAY I'm suffering through the middle books... so not only did those bad middle books "ruin" the early books, they've also ruined the books that came after.
Because I'll never know what happens with Rand, Matt, and uh.. .Pippin? And the various interchangable huffy women that hang around them that I can't remember the names of, and while I care a little, not enough.