It's always funny how long-running shows forget their initial premise the longer they run. To shoehorn in more and more outrageous plots, they have to break their own rules from earlier seasons, and this usually passes without the slightest comment.

Roseanne might have been the most egregious, with the well-known insanity of the final season of its initial run.

But also Little House on the Prairie forgot it was on the frontier in later seasons, becoming less about historical hardship and more about preaching religious morality.

*M*A*S*H avoided a lot of this, but its later seasons were also rather inconsistent... One example is when the crew manages to get a tape recorder and film to the States and back in time for one character's anniversary... despite seasons of jokes about how slow the mail is.

So many more examples spring to mind...