I'm going to change the question slightly. What topics do we all think are taught in undergraduate mathematics, but often leak out of the curriculum so that students see too little of them? I have in mind the standard situation at large research universities, where there is a mix of good and not-so-good students.
My pet peeves:
- Complex numbers as they should appear in standard calculus and linear algebra. They tend to be postponed to upper division courses. Complex numbers greatly simplify both trig identities and partial fractions, but calculus students aren't told.
- Complex analysis. It tends to float to the top of upper division and disappear.
- Full multivariate calculus: The Jacobian of a general change of coordinates, the derivative of a multivariate inverse function, maybe also the multivariate Newton's method. The calculus sequence often chickens out and just does special cases of the first of these.
- Higher-dimensional Euclidean geometry. Like, the definition of an n-cube and the fact that it has 2n vertices.
- Multivariate probability, especially with both discrete and continuous features.