This rule applies to everyone I know. It really irritates me when people break this rule.
The Sequel Rule:
You cannot buy a sequel to a movie or a video game without owning the original. If you see the sequel at a price you can't pass up with no possibility of seeing a deal this good again, you can buy it, but only if you buy the original before you get home.
The exception is sequels that do not continue the same storyline such as most Final Fantasy games and Halloween III.