I read that Morris had suggested the theoretical possibility of an Initial Sequence Number attack in Bell Labs Computer Science Technical Report #117, February 25, 1985. In 1995, Kevin Mitnick carried out this attack (described in Tsutomu Shimomura's book Takedown and in many other resources online). I find it very interesting that security problems so often go unfixed until after they're exploited.
I had this experience once as a system administrator in the CS department. I had noticed that a course's submission system was insecure and that students could steal from and/or overwrite other students' submissions. I emailed the professor about this in November 2003 and again in February 2004 when the problem still had not been fixed. In March 2005, a student was caught cheating, and it turned out that the student had exploited the security problem that I had reported more than a year earlier. If the professor had taken half an hour to carry out the single-command fix that I had proposed, then this student may not have cheated.
Another recent example of this is website encryption. For years, it has been common knowledge that unencrypted HTTP sessions can be hijacked. However, almost no major websites used SSL by default. Finally, Firesheep made people realize that this was a real problem (even though it had already been a real problem for years earlier). Somehow, people feel justified in ignoring security problems if they think the exploit sounds hard or unlikely, even if it is neither.