I was thinking about all the tv shows that I like and it seems something usually goes wrong in the 5th season.

The Brady Bunch: The Brady's last season when Oliver showed up, they were up against Sanford and Son, Robert Reed gets in a full-blown arguement with producers and refuses not only to be in the episode but also refuses to come back.

Three's company: Suazanne's Somers' really, really gay salary dispute gets her fired and kicked off the show.

Roseanne: Lecy Goranson goes away to college and off goes Becky number one.

The 5th season was also the final season for the Wonder Years. Can you think of any others?

Season 5 of I Dream of Jeannie was the last season. Tony and Jeannie got married that season. They decided not to continue the show because the ratings were low and since everyone knew about Jeannie and she and Tony were married, it took away the core of the show.
The core of the show was the fact that Jeannie was a secret and she and Tony had sexual tension, once she and Tony were married, everyone knew about her so the show lost it's appeal.
I was kind of upset that they didn't try to keep the show on the air at least one more season, but it was the early '70's and sitcoms were changing and fantasy shows were on the wayside by then. Oh, well. :(

well typically by the 5th year shows have run their course... most tv shows have a shelf life, or a period of time in which people loose interest; and 5 seems to be that magic number... even longer running shows loose steam by the 5th season.

I don't agree with this. Season 5 was the BEST season of Friends.

The 5th season is also the time when series hit the magic 100 episode mark, which is the normal amount that is needed to score a syndication deal with various stations. Once the cast and crew realize this, the quality of the episodes begin to drop off to a point where they inevitably reach the shark-jumping point. To keep their show on the air a little longer, they'll try to experiment and make changes to see what works and what doesn't: adding new characters, getting rid of old ones, making one of the characters pregnant, having someone die, etc. Usually, these moves don't work but it doesn't matter because the cast is now set for life so why should they care---and, for that matter, why should we?