My polyrepo cautionary tale: Two repos, one for fooclient, one for fooserver, talking to each other over protocol. Fooserver can do scary dangerous permanent things to company server instances, of which there are thousands.

Fooserver sprouts a query syntax ("just do this for test servers A and B"), pushed to production. Fooclient sprouts code that relies on this, pushed to production. A bit later, Fooserver is rolled back, blowing away query syntax, pushed to production. "Just do this for test servers A and B" now becomes "Do this for every server in the company". Hilarity ensues.