See my responses to a sibling comment for Github bug reports. I had a great experience with them: very fast triage and fixes, plus a lot of help developing a workaround so we could target a range of Racket versions. (Distros still ship some old versions!)

These bugs were found while working on Herbie <http://herbie.uwplse.org/>. We use the FFI to bind to your machine's math libraries so we can properly evaluate how accurate a floating-point expression is on your machine.