What I mean is that order `k` B-splines are by construction `k` times differentiable. This is different from the piecewise polynomials, which can have discontinuities of any kind between the intervals. Of course, now that I think about it more, it's possible to just add `k` duplicate knots to the spline at each pp breakpoint to reproduce the exact piecewise polynomials that came in as input. So no warning heuristics necessary in the end.

For the conversion-to-spline step, it would however be useful to have a "simplification" step, which looks at the continuity of the function based on some user-specified numerical tolerance, and reduces the number of knots as far as possible.