1. Character A overhears character B's conversation with character C and misinterprets it to mean that character D is pregnant/dead/gay/the result of a sex-change/richer than he really is/poorer than he really is/in love with character A/in hate with character A/or some other equally silly idea. Character A then totally over reacts to this knowledge and "hilarity" ensues. I think every second _Three's Company_ episode used this plot.
- Relatives come to visit.
- Wife wants to get a job.
4. Husband/Wife thinks wife/husband is having an affair, develops elaborate scheme to catch her/him in the act. It turns out to be something innocent.
5. Wife/kid breaks/kills something and tries to find something to replace it.
And now for some blathering: I don't think using these plots or others like them is the kiss of death for a show. Some shows like _The Simpsons_ or _I Love Lucy_ have so much going for them in sight gags and comic characters that the plot isn't all that makes the show.