a camel's nose (under the tent)

A small, seemingly innocuous act or decision that will lead to much larger, more serious, and less desirable consequences down the line. The term refers to an alleged Arab proverb that if a camel is allowed to get its nose inside of a tent, it will be impossible to prevent the rest of it from entering. Some regard legalizing same-sex marriage as a camel's nose under the tent, eventually leading to the destruction of marriage altogether. Not hiring an exterminator at the first sign of termites has proved to be a camel's nose, as much of our woodwork is now destroyed.