Examples of abstractions which are not leaky?

The only ones I can think of are the ones that are considered more "true" than the underlying thing they abstract.

Example: Integers. Add 5 jugs of water with a gallon each and you end up with almost 5 gallons. You call that a measurement error, not a leaky abstraction. The jugs are wrong and the math is right. Play on words really, since you could as well say that the integers are a leaky abstraction that fails to take into account inexactness. Point is, people don't tend to think of it that way.