If we are given n real-valued smooth functions on R-2n which are in involution, then, under some mild hypotheses, the subset of R-2n where these functions are linearly independent is not simply connected.
Paiva, J. (2000). A topological property of integrable systems. Proceedings of the American Mathematical Society, 128(8), 2507-2508. https://doi.org/10.1090/S0002-9939-00-05783-X (Original work published 2000)