When working on pimsync[1] and the underlying WebDAV/CalDAV/CardDAV implementation in libdav, I wrote "live tests" early on. These are integration tests, which use real servers (radicale, xandikos, nextcloud, cyrus, etc). They do things like "create an event, update the event, fetch it, validate it was updated". Some test handle exotic encoding edge cases, or trying to modify something this a bogus "If-Match" header. All these tests were extremely useful to validate the actual behaviour, in great deal because the RFCs are pretty complex and easy to misinterpret. For anyone working on the field, I strong suggest having extensive and easy to execute integration tests with multiple servers (or clients).

All servers have quirks, so each test is marked as "fails on xandikos" or "fails on nextcloud". There's a single test which fails on all the test servers (related to encoding). Trying to figure out why this test failed drove me absolute crazy, until I finally understood that all implementations were broken in the same subtle way. Even excluding that particular test, all server fail at least one other test. So each server is broken in some subtle way. Typically edge-cases, of course.

By far, however, the worst offender is Apple's implementation. It seems that their CalDAV server has a sort of "eventual consistency" model: you can create a calendar, and then query the list of calendars… and the response indicates that the calendar doesn't exist! It usually takes a few seconds for calendars to show up, but this makes automated testing an absolute nightmare.

[1]: https://pimsync.whynothugo.nl/